CHAPTER VII.

THALLOPHYTA.

The divisions of the plant kingdom dealt with in the following chapters

of Volume I. are taken in their natural sequence, beginning with the

lowest and passing gradually to the highest groups. The list of the

classes and families included in Chapters VII.–XI. is given in the

table of contents preceding Chapter I.

Thallophytes are of the simplest type, but they exhibit a very wide

range as regards both the structure and differentiation of the

vegetative body and the methods of reproduction. In some cases the

individual consists of a minute simple cell which multiplies by

cell-division; in others the body or thallus is made up of a number of

similar units, while in a great number of forms there is a well-marked

physiological division of labour, as expressed both in the external

division of the thallus into distinct organs corresponding in function

to the root, stem, and leaves of the higher plants, and further in the

high degree of histological differentiation of the tissues. In other

thallophytes, again, the thallus is a _coenocyte_ either unseptate or

incompletely septate; that is, the individual consists of a single cell

differing from a true plant-cell, in the stricter sense of the term, in

possessing several nuclei, in other words, the thallus is divided up

into compartments by transverse septa, but each division contains more

than one nucleus. Such coenocytic plants may show well-marked external

differentiation of the thallus into members or parts subserving

different functions.

A similar wide range is covered by the methods of reproduction among

thallophytes.

I. PERIDINIALES.

The organisms included under this head are of little importance from a

palaeontological point of view, but a brief reference may be made to

them as a section of the Thallophyta.

The Peridiniales include very small single-celled organisms which have

often been described as occupying a position on the borderland between

animals and plants, lying on the “shadowy boundary between animal and

vegetable life.” The individuals are rarely naked, more frequently

they are covered with a cellulose or mucilaginous investment which

has frequently the form of two or more minute armour-like plates of

a limiting membrane. The chromatophores are green, yellow, brown or

colourless. Simple division is the usual method of reproduction, but

spores have been described as occurring in some species. The motile

forms are provided with cilia. The Peridiniaceae, a section of the

Peridiniales, are regarded as nearly related to the Diatoms.

The Peridiniales play an important rôle in the Plankton flora of the

sea and freshwater lakes, and have a world-wide distribution. In the

narrative of the Challenger cruise they are described as occasionally

filling the tow-nets with a yellow coloured slime[165]. Some genera,

such as _Ceratium_, are found in enormous numbers off the British coast.

As an example of the occurrence of fossil representatives of the

Peridiniaceae reference may be made to one of two species of

_Peridinium_ described by Ehrenberg in 1836. These were found in a

siliceous rock described as Cretaceous in age from Delitzsch in Saxony.

A comparison of Ehrenberg’s figures of the fossil species _Peridinium

pyrophorum_ Ehrenb.[166], with those of the recent species _Peridinium

divergens_ Ehrenb., as given by Schütt[167] and other writers, brings

out clearly the very close resemblance if not identity of the two

forms. Bütschli[168] in his account of the Dinoflagellata in Bronn’s

_Thier-Reich_ confirms Ehrenberg’s determination of _Peridinium

pyrophorum_, and points out its striking agreement with the recent

species.

II. COCCOSPHERES AND RHABDOSPHERES.

(Organisms of doubtful affinity.)

Our knowledge of these minute calcareous organisms is derived from

Huxley’s description of coccoliths from the Atlantic in 1857, and from

the accounts of Wallich, John Murray, and other writers. In the first

volume of the narrative of the Challenger cruise[169] and in the volume

on deep-sea deposits[170] these minute forms of life are figured and

described. In the latter volume both genera are spoken of as extremely

abundant in the surface waters of the tropical and temperate regions

of the open ocean, and as forming an important constituent of the

Globigerine ooze; they are said to occur entangled in the gelatinous

substance of the Radiolarians, Diatoms, and Foraminifera, and are very

common in the stomachs of Salps, Pteropods and other pelagic animals.

Rhabdospheres are rare in regions where the temperature of the water

sinks below 65° F.; the Coccospheres occur in tropical and temperate

latitudes, and extend further north and south than the Rhabdospheres.

As regards their botanical position, John Murray expresses the view

that they are in all probability pelagic algae.

In the interesting memoir by Schütt on the _Pflanzenleben der

Hochsee_[171] there occurs a short reference to the forms described

in the Challenger Reports, but they were not obtained by the staff

of the Hensen Plankton Expedition and Schütt’s remarks are not based

therefore on personal observations. While admitting the existence of

such bodies, he points out that Zoologists have referred Coccospheres

and Rhabdospheres to the algae as organisms which cannot be included

in any group of animals, and Schütt is unable to recognise a sufficient

reason for referring them to this class of plants. It is suggested

indeed that they may be purely inorganic structures.

[Illustration: FIG. 25. (From Murray and Blackman).

_A_, Coccosphere × 1300. _B_, Rhabdosphere × 900. _C_, Portion of

the same × 1300. _D_, Rhabdosphere of another type, in optical

section × 1900. _E_, The same in surface view × 1900. _F_, End of

one of the trumpet-shaped appendages of _E_.]

The most recent account of these two genera is by Messrs G. Murray and

Blackman a short notice in _Nature_ for April 1, 1897[172]. Numerous

examples of Coccospheres and Rhabdospheres were obtained by Capt.

Milner of the R.M.S. Para during a voyage to Barbados by allowing

the sea water to enter the feed-pipe of the boiler through a fine

muslin net. All the forms described in the Challenger Reports were met

with, and an examination of the material by means of extremely high

objectives has confirmed the original account of the genera, and added

some points to our previous knowledge.

_Coccospheres_ (fig. 25 _A_). Spherical bodies of exceedingly small

size, consisting of a central protoplasmic vesicle covered with

overlapping circular calcareous scales, each of which is attached to

the minute cell by a button-like projection. The scales are frequently

found detached and are then spoken of as Coccoliths.

_Rhabdospheres_ (fig. 25 _B_–_F_). Spherical bodies, extremely minute,

consisting of a single cell, on the surface of which are embedded

numerous calcareous plates bearing long blunt spines (fig. 25, _C_)

or beautiful trumpet-like appendages (fig. 25, _D_–_F_). The detached

plates of Rhabdospheres are known as Rhabdoliths.

In addition to the text-figures of Coccospheres and Rhabdospheres in

the Challenger Reports, the same structures are shown in samples of

globigerine ooze figured in Plate XI. of the Monograph on deep-sea

deposits. In a recent number of _Nature_ Messrs Dixon and Joly[173]

have announced the discovery of Coccoliths and Coccospheres in the

coastal waters off South County Dublin. They estimate that in one

sample of water taken about three miles from the Irish coast there were

200 Coccoliths in each cubic centimetre of sea water.

The interest of these calcareous bodies from a palaeobotanical point of

view lies in the fact that similar forms have been recognized in the

Chalk and the Upper Lias. Sorby, in his memorable Address delivered

before the Geological Society in 1879, refers to the abundance of

Coccoliths in sections of chalk which he examined[174]. Rothpletz[175]

has recently recorded the occurrence of numerous Coccoliths, 5–12

µ in diameter, associated with the skeleton of a horny sponge

(_Phymatoderma_) of Liassic age.

The question of the nature of Coccospheres and Rhabdospheres cannot be

regarded as definitely settled. It has been shown by J. Murray, and

more recently by G. Murray and V. H. Blackman, that on the solution of

the calcareous material by a weak acid there remains a small gelatinous

body apparently protoplasmic in nature. We may at least express the

opinion that Schütt’s suggestion as to their being inorganic must be

ruled out of court. It would appear that they are extremely minute

unicellular organisms characterised by a delicate calcareous armour

consisting of numerous plates or scales. We know nothing as to their

life-history, and cannot attempt to determine their affinities with

any degree of certainty until further facts are before us. It is not

improbable that they are algae of an extremely minute size, and the

evidence so far obtained would lead us to regard them as complete

individuals rather than the reproductive cells of some larger organism.

Mr George Murray is of opinion that they are certainly algae, but he

considers that they cannot be included in any existing family. It is

conceivable that they may be minute eggs or reproductive cells of

animals or plants, but on the whole the balance of probability would

seem to be in favour of regarding them as autonomous organisms.

III. SCHIZOPHYTA.

A. SCHIZOPHYCEAE (CYANOPHYCEAE).

B. SCHIZOMYCETES (Bacteria).

In this group are included small single-celled plants of an extremely

low type of organisation, in which reproduction takes the form of

multiplication by simple cell-division, or the formation of spores.

The characteristic method of reproduction by division has given rise

to the general term Fission-plants for this lowest sub-class in the

vegetable kingdom. In many cases the members of this sub-class contain

chlorophyll, and associated with it a blue-green colouring matter; such

plants are classed together as the Blue-green algae, Cyanophyceae,

or Schizophyceae. Others, again, are destitute of chlorophyll, and

may be conveniently designated Schizomycetes or Fission-fungi. Seeing

how close is the resemblance and relationship between the members of

the sub-class, it has been the custom to include them as two parallel

series under the general head, Schizophyta, rather than to incorporate

them among the Algae and Fungi respectively.

A. SCHIZOPHYCEAE (CYANOPHYCEAE or Blue-green Algae).

_Chroococcaceae._ Thallus of a single cell, the cells may be either

free, or more usually joined together in colonies enveloped by a common

gelatinous matrix, formed by the mucilaginous degeneration of the outer

portion of the cell-walls. Reproduction by means of simple division or

resting cells.

_Nostocaceae._ Thallus consists of simple or branched rows of cells in

which special cells known as _heterocysts_ often occur. Reproduction

by means of germ-plants or _hormogonia_, or by resting cells specially

modified to resist unfavourable conditions.

In both families the individuals are surrounded by a gelatinous

envelope, which in some genera assumes the form of a conspicuous

and comparatively resistant sheath. Marine, freshwater, and aerial

forms are represented among recent genera. Several species occur as

endophytes, living in the tissues or mucilage-containing spaces in the

bodies of higher plants. In addition to the frequent occurrence of

blue-green algae in freshwater streams and on damp surfaces, certain

forms are particularly abundant in the open sea[176], and in lakes or

meres[177] where they are the cause of what is known in some parts

of the country as “the breaking of the meres” (“Fleurs d’eau”). From

the narrative of the cruise of the Challenger, we learn that the

Oscillariaceae are especially abundant in the surface waters of the

ocean. The “sea sawdust” so named by Cook’s sailors[178], and the same

floating scum collected by Darwin[179], affords an illustration of the

abundance of some of these blue-green algae in the sea.

Another manner of occurrence of these plants has been recorded by

different writers, which is of special importance from the point of

view of fossil algae. On the shores of the Great Salt Lake, Utah,

there are found numerous small oolitic calcareous bodies thrown up

by the waves[180]. These are coated with the cells of _Glœocapsa_

and _Glœotheca_, two genera of the Chroococcaceae. Sections of the

grains reveal the presence of the same forms in the interior of the

calcareous matrix, and it has been concluded, on good evidence that the

algae are responsible for the deposition of the carbonate of lime of

the oolitic grains. By extracting the carbonic acid which they require

as a source of food, from the waters of the lake, the solvent power of

the water is decreased and carbonate of lime is thrown down. In similar

white grains from the Red Sea[181] there is a central nucleus in the

form of a grain of sand, and cells of Chroococcaceae occur in the

surrounding carbonate of lime as in the Salt Lake oolite. Prof. Cohn of

Breslau in 1862 demonstrated the importance of low forms of plant life

in the deposition of the Carlsbad “Sprudelstein[182].” On the bottom of

Lough Belvedere, near Mullingar in Ireland[183], there occur numerous

spherical calcareous pebbles, of all sizes up to that of a filbert.

From a pond in Michigan (U.S.A.)[184] similar bodies have been obtained

varying in diameter from one to three and a half inches. In the former

pebbles a species of _Schizothrix_, one of the Nostocaceae occurs

in abundance, in the form of chains of small cells enclosed in the

characteristic and comparatively hard tubular sheath, and associated

with _Schizothrix fasciculata_ there have been found _Nostoc_ cells and

the siliceous frustules of Diatoms. In the Michigan nodules the same

_Schizothrix_ occurs, associated with _Stigonema_ and _Dichothrix_,

other genera of the Nostocaceae. One of the Michigan pebbles is shown

in section in fig. 32 _D_.

[Sidenote: OOLITIC STRUCTURE.]

The connection between the well-known oolitic structure, characteristic

of rocks of various ages in all parts of the world, and the presence

of algal cells is of the greatest interest from a geological point

of view. In recent years considerable attention has been paid to

the structure of oolitic rocks, and in many instances there have

been found in the calcareous grains tubular structures suggestive of

simple cylindrical plants, which have probably been concerned in the

deposition of the carbonate of lime of which the granules consist. In

1880 Messrs Nicholson and Etheridge[185] recorded the occurrence of

such a tubular structure in calcareous nodules obtained from a rock

of Ordovician age in the Girvan district of Scotland. These Authors

considered the tubes to be those of some Rhizopod, and proposed to

designate the fossil _Girvanella_.

_Girvanella_ (fig. 26).

Messrs Nicholson and Etheridge defined the genus as follows:—

“Microscopic tubuli, with arenaceous or calcareous (?) walls,

flexuous or contorted, circular in section, forming loosely

compacted masses. The tubes, apparently simple cylinders, without

perforations in their sides, and destitute of internal partitions

or other structures of a similar kind.”

[Illustration: FIG. 26. _Girvanella problematica_, Eth. and Nich.

Tubules of _Girvanella_ lying in various positions and surrounding

an inorganic ‘nucleus’ or centre. From a section of Wenlock

limestone, May Hill. × 65]

Since this diagnosis was published very many examples of similar

tubular fossils have been described by several writers in rocks from

widely separated geological horizons. The accompanying sketch (Fig.

26), drawn from a micro-photograph kindly lent to me by Mr Wethered

of Cheltenham, who has made oolitic grains a special subject of

careful investigation, affords a good example of the occurrence of

such tubular structures in an oolitic grain of Silurian age from

the Wenlock limestone of May Hill, Gloucestershire[186]. In the

centre is a crystalline core or nucleus round which the tubules have

grown, and presumably they had an important share in the deposition

of the calcareous substance. The nature of _Girvanella_, and still

more its exact position in the organic world, is quite uncertain;

it is mentioned rather as _à propos_ of the association of recent

Cyanophyceae with oolitic structure, than as a well-defined genus of

fossil algae.

In the typo description of the calcareous nodules from Michigan, Murray

speaks of the _Schizothrix_ filaments at the surface of the pebbles

as fairly intact, while nearer the centre only sheaths were met with.

It is conceivable that in some of the tubular structures referred to

_Girvanella_ we have the mineralised sheaths of a fossil Cyanophyceous

genus[187]. The organic nature of these tubules has been a matter

of dispute, but we may probably assume with safety that in some at

least of the fossil oolitic grains there are distinct traces of some

simple organism which was in all likelihood a plant. Some authors

have suggested that _Girvanella_ is a calcareous alga which should be

included in the family Siphoneae[188]. As a matter of fact we must

be content for the present to leave its precise nature as still _sub

judice_, and while regarding it as probably an alga, we may venture

to consider it more fittingly discussed under the Schizophyta than

elsewhere.

Wethered[189] would go so far as to refer oolitic structure in general

to an organic origin. While admitting that a Girvanella-like structure

has been very frequently met with in oolitic rocks, it would be unwise

to adopt so far-reaching a conclusion. It is at least premature to

refer the formation of all oolitic structure to algal agency, and the

evidence adduced is by no means convincing in every case. The discovery

of _Girvanella_ and allied forms in rocks from the Cambrian[190],

Ordovician, Silurian, Carboniferous, Jurassic and other systems is a

striking fact, and lends support to the view that oolitic structure

is in many cases intimately associated with the presence of a simple

tubular organism. Among recent algae we find different genera, and

representatives of different families, growing in such a manner and

under such circumstances as are favourable to the formation of a

ball-like mass of algal threads, which may or may not be encrusted

with carbonate of lime. Similarly as regards oolitic grains of various

sizes, and the occurrence in rocks of calcareous nodules, the tubular

structure is not always of precisely the same type, and cannot always

be included under the genus _Girvanella_.

Several observers have recorded the occurrence of low forms of

plant-life in the waters of thermal springs. It has been already

mentioned that Cohn described the occurrence of simple plants in the

warm Carlsbad Springs, and fission-plants of various types have been

discovered in the thermal waters of Iceland, the Azores[191], New

Zealand, the Yellowstone Park, Japan, India, and numerous other places.

A few years ago Mr Weed, of the geological survey of the United States,

published an interesting account of the formation of calcareous

travertine and siliceous sinter in the Yellowstone Park district[192].

This author emphasizes the important rôle of certain forms of plants

in the building up of the calcareous and siliceous material. Among

other forms of frequent occurrence, _Calothrix gypsophila_ and a

species _Leptothrix_ are mentioned, the former being a member of

the Nostocaceae, allied to _Rivularia_, and the latter a genus of

Schizomycetes. In many of the springs there are found masses of algal

jelly like those previously described by Cohn in the Carlsbad waters.

Sections of such dried jelly showed a number of interlaced filaments

with glassy silica between them. Weed refers to the occurrence of small

gritty particles in this mucilaginous material. These are calcareous

oolitic granules which are eventually cemented together into a compact

and firm mass of travertine by the continued deposition of carbonate

of lime. The presence of the plant filaments is often difficult to

recognise in the “leathery sheet of tough gelatinous material,” or in

“the skeins of delicate white filaments” which make up the travertine

deposits.

[Sidenote: BORINGS IN SHELLS.]

Under the head of _Cyanophyceae_, mention should be made of the recent

genus _Hyella_[193], which occurs as a perforating or boring alga in

the calcareous shells of molluscs. On dissolving the carbonate of

lime of shells perforated by this alga, the latter is isolated and

appears to consist of rows of small cells, with possibly some sporangia

containing spores. Other boring algae have been recorded among the

Chlorophyceae, and recently a member of the Rhodophyceae[194] has been

found living in the substance of calcareous shells. Such examples are

worthy of note in view of the not infrequent occurrence of fossil

corals, shells and fish-scales, which have evidently been bored by an

organism resembling in form and manner of occurrence these recent algal

borers.

The occurrence of small ramifying tubes in recent and fossil corals,

fish-scales, and bones was long ago pointed out by Quekett[195],

Kölliker[196], Rose[197] and other writers[198]. These narrow tubular

cavities have generally been attributed to the boring action of

some parasitic organism, either a fungus or an alga. In 1876 Duncan

published two important papers[199] dealing with the occurrence of

such tubes in recent corals, as well as in the calcareous skeleton

of _Calceolina_, _Goniophyllum_ and other Palaeozoic, Mesozoic and

Tertiary species of corals. This writer attributed the formation of the

cavities in the case of the fossil species to the action of a fungus

which he named _Palaeachlya perforans_, and considered as very nearly

related to _Achlya penetrans_ found in the “dense sclerenchyma” of

recent corals. In fig. 27 A. is reproduced one of the drawings given

by Rose[200] in his paper published in 1855; it shows a section of a

fish-scale from the Kimeridge clay which has been attacked by a boring

organism. Rose attributes the dichotomously branched canals to some

“infusorial parasite.”

[Illustration: FIG. 27. A, Section of a fish-scale from the Kimeridge

Clay, showing branched canals, made by a boring organism, × 85.

B, Section of a Solen shell, penetrated in all directions by the

boring thallus of _Ostracoblabe_ (a fungus?), × 330. C, Piece of

the thallus of _Ostracoblabe_ isolated by decalcification, × 745.

A, after Rose. B and C, after Bornet and Flahault.]

In the important paper by MM. Bornet and Flahault on perforating algae

a full description is given of various boring forms belonging to the

Chlorophyceae and the Cyanophyceae[201]. The canals which these algae

produce in calcareous shells and other hard substances are of the same

type as those previously described in fossil corals, fish-scales and

bones. In dealing with living perforating Thallophytes the colour and

other cell-contents often enable us to distinguish between algae and

fungi, but in fossil specimens such tests cannot be applied. The fossil

tubular borings may or may not show traces of the transverse septa

and reproductive cells; it is often the case that no trace of the

organism has been left, but only the canals by which it penetrated the

calcareous or bony skeleton. In some of the examples of _Palaeachlya_

figured by Duncan there appear to be numerous spores in some of the

sections, but it is generally a very difficult and often an impossible

task to discriminate between the borings of fungi and algae in fossil

material.

Fig. 27 B, which is copied from one of Bornet and Flahault’s drawings,

represents a piece of Solen shell riddled with small canals made by

the organism which has been named by the French authors _Ostracoblabe

implexa_, and regarded by them as a fungus. Fig. 27 C represents a

small piece of the vegetative body of _Ostracoblabe_ obtained from a

decalcified shell. In endeavouring to determine the organism which

has produced borings in fossil corals or shells, it must be borne in

mind that some forms of canals or passages may have been the work of

perforating sponges, but these are larger in diameter than those made

by algae or fungi. By some writers[202] the tubular cavities in shells

have been referred to true algae, but others consider them to be of

fungal origin.

As an example of a fossil alga referred to the Cyanophyceae, the genus

_Zonatrichites_[203] may be quoted. Bornemann, who first described the

specimens, points out the close resemblance in habit to some members of

the recent Rivulariaceae.

_Zonatrichites._

The author of the genus defines it as follows:—

“A calcareous alga, with radially arranged filaments, forming

hemispherical or kidney-shaped layers, growing on or enclosing

other bodies. Parallel or concentric zones are seen in

cross-section, formed by the periodic growth of the alga, the

older and dead layers serving as a foundation on which the young

filaments grow in radially arranged groups.”

The nodules which are apparently formed by species of this genus occur

in various sizes and shapes; Bornemann describes one hemispherical mass

8 cm. broad and 4 cm. thick. In some cases the organism has given rise

to oolitic spherules, which in radial section exhibit the branched

tubular cells spreading in fan-shaped groups from the centre of the

oolitic grain. The section parallel to the surface of a nodule presents

the appearance of a number of circular or elliptical tubes cut across

transversely or more or less obliquely. The resemblance between the

fossil and a specimen of the recent species _Zonatrichia calcivora_

Braun, is certainly very close, but it is very difficult, in the

absence of material exhibiting more detailed structure than is shown in

the specimens described by Bornemann, to decide with any certainty the

true position of the fossil. The figures do not enable us to recognise

any trace of cells in the radiating tubes. It is possible that we

have in _Zonatrichites_ an example of a Cyanophyceous genus in which

only the sheaths of the filaments have been preserved. In any case it

is probable that this Mesozoic species affords another instance of a

fossil alga which has been responsible for certain oolitic or other

structures in limestone rocks.

The species described by Bornemann was obtained from a Breccia near

Lissau in Silesia, of Keuper age.

M. Renault has recently described certain minute structures in

a Palaeozoic coprolite to which he gives the name _Gloioconis

Borneti_[204], and which he regards as a Permian gelatinous alga

similar to the well-known recent genus _Glœocapsa_. The appearances

revealed in a section of the coprolite are interpreted by this author

as a collection of small colonies of a unicellular gelatinous alga in

various stages of development. Renault’s figure shows a spherical group

of faintly outlined and cloudy bodies, most of which include one or two

small dark spots. The latter are regarded as the cells of the alga, and

the surrounding cloudy substance is described as the gelatinous sheath.

The absence of a nucleus in these extremely minute fossil cells (8–10

µ in diameter) is referred to as an argument in favour of referring

the organism to the Cyanophyceae rather than to the Chlorophyceae. It

is possible that the ill-defined structure described by Renault may be

a petrified alga, but there is not sufficient evidence to warrant a

decided opinion; the absence of nuclei can hardly be taken seriously

in such a case as this as an argument in favour of the Cyanophyceae.

[Sidenote: CYANOPHYCEAE.]

Although our exact knowledge of fossil Cyanophyceae is extremely small,

it is probable that such simple forms of plants existed in abundance

during the past ages in the earth’s history. Several writers have

expressed the opinion that the blue-green algae may be taken as the

modern representatives of those earliest plants which first existed

on an archaean land-surface. The living species possess the power of

resisting unfavourable conditions in a marked degree, and are able

to adapt themselves to very different surroundings. Their occurrence

in hot springs proves them capable of living under conditions which

are fatal to most plants, and suggests the possibility of their

occurrence in the heated waters which probably constituted the medium

in which vegetable life began. An interesting example of the growth of

blue-green algae under unfavourable conditions was recorded in 1886 by

Dr Treub[205] of the Buitenzorg Gardens, Java. In 1883 a considerable

part of the island Krakatoa, situated in the Straits of Sunda, between

Sumatra and Java, was entirely destroyed by a terrific volcanic

explosion. What remained had been reduced to a lifeless mass of hot

volcanic ashes. Three years later, Treub visited the island, and found

that several plants had already established themselves on the volcanic

rocks. Various ferns and flowering plants were recorded in Treub’s

description of this newly established flora. It seemed that the barren

rocky surface had been prepared for the more highly organised plants by

the action of certain forms of Cyanophyceae, which were able to live

under conditions which would be fatal to more complex types.

In the petrified tissues of fossil plants there are occasionally

found small spherical vesicles, with delicate limiting membranes, in

the cavities of parenchymatous cells or in the elements of vascular

tissue. Some of these spherical inclusions have been described as

possibly simple forms of endophytic algae[206], such as we are now

familiar with in species of the Cyanophyceae and other algae. So far,

however, no recorded instance of such fossil endophytic algae is

entirely satisfactory. Some of the cells figured by Williamson as

possibly algae, endophytic in the tissues of Coal-Measure plants, are

no doubt thin-walled vesicles which formed part of a highly vacuolated

cell-contents. Examples of such vesicles in living and fossil cells are

shown in fig. 42. The fact that the contents of living plant tissues

have been erroneously described as endophytic organisms, should serve

as a warning against describing fossil endophytes without the test of

good evidence to support them.

The description of a fossil _Nostoc_ by the late Prof. Heer[207] from

the Tertiary rocks of Switzerland cannot be accepted as a trustworthy

example of a fossil plant, much less of a genus of recent algae. The

application of recent generic names to fossils which are possibly not

even organic must do more harm than good.

B. SCHIZOMYCETES (Bacteria).

It is impossible to draw a sharp line between the two subdivisions of

the Schizophyta. The so-called Fission-Fungi or Bacteria differ from

the Schizophyceae or Fission-Algae in the cell-contents being either

colourless, blood-red or green, but never blue-green. We may regard

the Bacteria, generally, as the lowest forms of plants; they are

extremely simple organisms which have been derived from some primitive

types which possessed the power of independent existence and contained

chlorophyll—that important substance which enables a plant to obtain

its carbon first-hand from the carbon dioxide of the atmosphere.

Bacteria may be briefly described as single-celled plants, and as de

Bary suggested comparable in shape to a billiard ball, a lead pencil or

a corkscrew[208]. A single spherical or cylindrical cell measures about

1 µ in diameter[209]. They occur either singly or in filaments, or as

masses of various shapes consisting of numberless bacterial cells. The

nature and manner of life of Bacteria, and their extraordinary power

of successfully resisting the most unfavourable conditions, render it

probable that they constitute an extremely ancient group of organisms.

The wonderful perfection of preservation of many fossil plants enables

us to investigate the contents of petrified cells and to examine in

minutest detail the histology of extinct plants. To those who are

familiar with the possibilities of microscopical research as applied to

silicified and calcified fossil tissues, it is by no means incredible

that evidence has been detected of the existence of Bacteria as far

back in the history of the earth as the Carboniferous and Devonian

periods.

Were there no trustworthy records of the occurrence of Bacteria in

Palaeozoic times, it would still be a natural supposition that these

ubiquitous organisms must have been abundantly represented. It has

been suggested as a probable conclusion that some forms of Bacteria,

which produced chemical changes in the soil necessary for the nutrition

of plants, must have existed contemporaneously with the oldest

vegetation[210].

The paper-coal of Toula, which in some places reaches a thickness of 20

cm., is a plant-bed of exceptional interest. It differs from ordinary

coal in being made up of numberless thin brown-papery sheets associated

with a darker coloured substance largely composed of ulmic acid. Prof.

Zeiller[211], in an interesting account of the papery layers, has

shown that they consist of the cuticles of a Lepidodendroid plant,

_Bothrodendron_. An examination of a piece of one of the sheets at

once reveals the existence of a regular network of which the walls of

the meshes are the outlines of the epidermal cells, the meshes being

bridged across by a thin light brown membrane which represents the

layer of cuticularised cell-wall of each epidermal cell. At regular

intervals and disposed in a spiral arrangement, we find small gaps

in the papery cuticle which mark the position of the _Bothrodendron_

leaves. These Palaeozoic cuticles are not petrified; they are only

slightly altered, and have retained the power of swelling in water,

being able to take up stains like recent tissues. It may reasonably

be assumed that the persistent cuticles owe their preservation to a

greater power of resistance to destructive agents than was possessed

by the other tissues of the plant. It is by no means unlikely, as

Renault[212] has recently suggested, that as the _Bothrodendron_

stem-fragments lay in the swamps or marshes the tissues were gradually

eaten away by Bacteria, but the cuticles successfully resisted the

attacks of the bacterial saprophytes. The same observer has described

what he regards as the actual organism which effected this wholesale

destruction, under the name _Micrococcus Zeilleri_. He finds, after

treating the cuticles with ammonia to remove the ulmic acid, that there

occur numerous minute spherical bodies, each surrounded by a thin

envelope, either singly or in groups on the surface of the cuticular

membrane. These vary in size from ·5µ to 1µ in diameter. I have not

been able to detect any satisfactory proof of such _Micrococci_ in

specimens of the paper-coal which were treated according to Renault’s

method, but it is extremely probable that this unusual method of

preservation of stem-cuticles is the result of selective bacterial

action.

Renault believes that some of the minute spherulitic structures which

are seen in sections of decayed tissues of Palaeozoic plants owe their

origin, in part, to the ravages of bacteria. The disorganisation of

parenchymatous cells gives rise to a gelatinous substance in which

needle-like crystals of silica may be deposited, from a siliceous

solution, in a matrix which has resulted from bacterial activity. In

some of the sections of tissues figured by Renault[213] the outlines of

a few cells are still indicated by fragments of the partially decayed

wall, while in other cells the walls have been completely destroyed by

Bacteria of which some are preserved in the centre of the cell-area,

forming a kind of nucleus to the siliceous spherulites.

[Sidenote: BACILLI.]

In addition to the _Micrococcus_ described by Renault from the Toula

paper-coal, there are a host of other forms which have been minutely

diagnosed and figured by Profs. Renault and Bertrand[214]. These

authors have discovered what they believe to be well-defined species of

_Micrococcus_ and _Bacillus_ ranging in age from Devonian to Jurassic.

The material which has afforded the somewhat startling results of

their investigations consists partly of the coprolites of reptiles and

fishes, and of silicified and calcified plant tissues.

_Bacillus Permicus._ Ren. and Bert.[215] (Fig. 28 B.)

This _Bacillus_, which was discovered in sections of a Permian

coprolite from Central France, has the form of cylindrical rods

12–14µ in length, and 1·3–1·5µ broad, rounded at each end. The rods

occur either singly or occasionally, two or three individuals are

joined end to end. Fig. 28 B represents a piece of one of Renault and

Bertrand’s sections; the small rods are clearly seen lying in various

directions in the homogeneous matrix of the coprolite. Each individual

is said to be surrounded by an extremely minute empty space ·4µ in

width, originally occupied by the Bacillus membrane, the central

rod representing the mineralised cell-contents. In this example the

petrifying substance was probably derived from the phosphate of calcium

of bones which were attacked by Bacteria. I am indebted to Prof.

Renault for an opportunity of examining specimens of this and other

fossil Bacteria, and in this particular case there is undoubtedly

strong evidence in favour of the author’s determination.

[Illustration: FIG. 28. A, _Bacillus Tieghemi_ Ren. and _Micrococcus

Guignardi_ Ren. B, _Bacillus Permicus_ Ren. (After Renault.)]

_Bacillus Tieghemi_ Ren.[216] and _Micrococcus Guignardi_ Ren.[217]

(Fig. 28 A.)

Renault has given the name _Bacillus Tieghemi_ to certain minute rods

6–10µ, in length, and 2·2–3·8µ broad, often containing a dark coloured

spherical spore-like body 2µ in diameter, which have been found in the

tissues of a Coal-Measure plant.

The name _Micrococcus Guignardi_ has been applied to more or less

spherical bodies 2·2µ in diameter, also met with in silicified plants.

A portion of one of Renault’s figures is reproduced in Fig. 28 A. The

faint and broken lines mark the position of the middle lamellae of

parenchymatous cells from the pith of a Calamite. The tissue has been

almost completely destroyed, but the more resistant middle lamellae

have been partially preserved. The short and broad rods represent what

Renault terms _Bacillus Tieghemi_; the small circle in the middle of

some of these being referred to as a spore, and in one specimen shown

in the figure, the second rod at right angles to the first is described

as a small daughter-Bacillus formed by the germination of the central

spore.

The isolated circles in the figure are referred to _Micrococcus_.

[Sidenote: FOSSIL BACTERIA.]

It is unnecessary to give an account of the numerous examples of

_Micrococci_ and _Bacilli_ described by Renault from Devonian,

Carboniferous, Permian and Jurassic rocks. We may, however, in a few

words consider the general question of the existence and possible

determination of fossil Bacteria.

In 1877 Prof. Van Tieghem[218] of Paris drew attention to the method

of operation and plan of attack of _Bacillus amylobacter_ as a

destructive agent in the decay of plant débris in water. He was able

to follow the gradual disorganisation of the tissues and the various

steps in the ‘butyric fermentation’ effected by this _Bacterium_.

Similarly the same author[219] was able to detect the action of an

allied organism in some silicified tissues from the Carboniferous

nodules of Grand-Croix, a well-known locality for petrified plants near

Saint-Étienne. He recognised also the traces of the _Bacillus_ itself

in the partially destroyed plant tissues. The Palaeozoic Bacteria

made use of some cellulose-dissolving ferment of which the action is

clearly demonstrated in sections of silicified tissues. Many of the

phenomena described by Renault and Bertrand as due to similar Bacterial

action, afford additional evidence that the gradual disorganisation of

vegetable tissues was effected in precisely the same manner as at the

present day.

In some cases we have I believe trustworthy examples of the Bacteria

themselves, both in coprolites and plant-tissues, but it is more

than probable that some of the recorded examples are not of any

scientific value. The examination of petrified tissues under the

higher powers of a microscope often reveals the existence of numerous

spherical particles and rod-like bodies which agree in shape with

_Micrococci_ or _Bacilli_. Minute crystals of mineral substances may

occur in the siliceous or calcareous matrix of a petrified plant

which simulate minute organic forms. Vogelsang[220] in his important

work _die Krystalliten_ has thrown considerable light on the ontogeny

of crystals, and the minute globulites and other forms of incipient

crystallisation might well be mistaken for Bacterial cells. Granting,

however, that we have satisfactory evidence, both direct and indirect,

that some forms of Bacteria lived in the decaying tissues of Palaeozoic

plants, and in the intestines of reptiles and other animals, we cannot

safely proceed to specific diagnoses and determinations[221].

Renault has pointed out that fossil Bacteria may often be more readily

detected than living forms owing to the presence of a brown ulmic

substance which results from the carbonisation of the protoplasm. He

is forced to admit, however, that such diagnostic characters as are

obtained by Bacteriologists by means of cultures cannot be utilised

when we are dealing with fossil examples! We are told that “Partout où

nous avons cherché des Bacteriaceés, nous en avons rencontré.”[222]

This indeed is the danger; an extended examination of fossil sections

under an immersion-lens must almost inevitably lead to the discovery

of minute bodies of a more or less spherical form which _might_ be

_Micrococci_. To measure, and name such bodies as definite species of

_Micrococci_ is, I believe, but wasted energy and an attempt to compass

the impossible.

Specialists tell us that the accurate determination of species of

recent Bacteria is practically hopeless: may we not reasonably conclude

that the attempt to specifically diagnose fossil forms is absolutely

hopeless? “The imagination of man is naturally sublime, delighted with

whatever is remote and extraordinary—”, but it is to be deplored if the

fascination of fossil bacteriology is allowed to warp sound scientific

sense.

IV. ALGAE.

A. DIATOMACEAE. (Diatoms.)

B. CHLOROPHYCEAE. (Green algae.)

C. RHODOPHYCEAE. (Red algae.)

D. PHAEOPHYCEAE. (Brown algae.)

The presence of chlorophyll is one common characteristic of the

numerous plants included in the Algae. The generally adopted

classification rests in part on an artificial distinction, namely the

prevailing colour of the plant.

It must be definitely admitted, at the outset, that palaeobotany has

so far afforded extremely little trustworthy information as to the

past history of algae. Were we to measure the importance of the

geological history of these plants by the number of recorded fossil

species, we should arrive at a totally wrong and misleading estimate.

By far the greater number of the supposed fossil algae have no claim

to be regarded as authentic records of this class of Thallophytes. It

has been justly said that palaeontologists have been in the habit of

referring to algae such impressions or markings on rocks as cannot well

be included in any other group. “A fossil alga,” has often been the

_dernier ressort_ of the doubtful student.

[Sidenote: LARGE SEAWEEDS.]

Before discussing our knowledge, or rather lack of knowledge, of

fossil algae at greater length, it will be well to briefly consider

the manner of occurrence and botanical nature of existing forms. In

the sea and in fresh water, as well as in damp places and even in

situations subject to periods of drought, algae occur in abundance in

all parts of the world. We find them attaining full development and

reproducing themselves at a temperature of −1° C. in the Arctic Seas,

and again living in enormous numbers in the waters of thermal springs.

Around the coast-line of land areas, and on the floor of shallow seas

algae exhibit a remarkable wealth of form and luxuriance of growth.

As regards habit and structure, there is every gradation from algae

in which the whole individual consists of a thin-walled unseptate

vesicle, to those in which the thallus attains a length unsurpassed by

any other plant, and of which the anatomical features clearly express

a well-marked physiological division of labour such as occurs in the

highest plants.

The large and leathery seaweeds which flourish in the extreme northern

and southern seas are plants which it is reasonable to suppose might

well have left traces of their existence in ancient sediments. Sir

Joseph Hooker, in his account of the Antarctic flora[223], investigated

during Sir James Ross’s voyage in H.M. ships Erebus and Terror, has

given an exceedingly interesting description of the gigantic brown

seaweeds of southern latitudes. The trunks are described as usually

5–10 feet long, and as thick as a human thigh, dividing towards the

summit into numerous pendulous branches which are again broken up into

sprays with linear ‘leaves.’ Hooker records how a captain of a brig

employed his crew for two bitterly cold days in collecting _Lessonia_

stems which had been washed up on the beach, thinking they were trunks

of trees fit for burning. On our own coasts we are familiar with the

common _Laminaria_, the large brown seaweed with long and strap-shaped

or digitate fronds which grows on the rocks below low-tide level. The

frond passes downwards into a thick and tough stipe firmly attached to

the ground by special holdfasts. A transverse section of the stalk of

a fairly old plant presents an appearance not unlike that of a section

of a woody plant. In the centre there is a well-defined axial region

or pith consisting of thick walled, long and narrow tubes pursuing a

generally vertical though irregular course, and embedded in a matrix

of gelatinous substance derived from the mucilaginous degeneration

of the outer portions of the cell-walls. The greater part of such a

section consists, however, of regularly disposed rows of cells which

have obviously been formed by the activity of a zone of dividing or

meristematic elements. The occurrence of distinct concentric rings in

this secondary tissue clearly points to some periodicity of growth

which is expressed by the alternation of narrow and broader cells. In

the Antarctic genus _Lessonia_, the stem reaches a girth equal to that

of a man’s thigh, and in structure it agrees closely with the smaller

stem of _Laminaria_. In these large algal stems, the cells are not

lignified as in woody plants, and in longitudinal section they have

for the most part the form of somewhat elongated parenchyma, differing

widely in appearance from the tracheids or vessels of woody plants. At

the periphery of the _Laminaria_ stem, represented in fig. 29, there

occur numerous and comparatively large mucilage ducts.

[Illustration: FIG. 29. A, Transverse section of the stipe of a

_Laminaria_, slightly enlarged. B, A small piece of the tissue

between the central ‘pith’ and ‘cortex’ showing the radially

disposed secondary elements more highly magnified.]

In certain algae of different families the thallus is encrusted with

carbonate of lime, and is thus rendered much more resistant. The

Diatoms, on the other hand, possess still more durable siliceous tests

which are particularly well adapted to resist the solvent action of

water and other agents of destruction. It is these calcareous and

siliceous forms which supply the greater part of the trustworthy data

furnished by fossil algae.

[Sidenote: SCARCITY OF FOSSIL ALGAE.]

It remains to consider some of the causes to which we may attribute

the scarcity of fossil algae, and the possible sources of error which

beset any attempt to describe or assign names to impressions and casts

simulating algal forms.

In the first place, the delicate nature of algal cells is a serious

obstacle to fossilisation. Even in plants in which the woody stems have

been preserved by a siliceous or calcareous solution, we frequently

find the more delicate cells represented by a mass of crystalline

matter without any trace of the cell-walls being preserved. In such

plants as algae, where the cell-walls are not lignified, but consist of

cellulose or some special form of cellulose, which readily breaks down

into a mucilaginous product, the tissues have but a small chance of

withstanding the wear and tear of fossilisation.

The danger of relying on external form as a means of recognition is

especially patent in the case of those numerous markings or impressions

frequently met with on rocks, and which resemble in outline the

thallus of recent algae. Among animals, such as certain Polyzoa, the

flat branching body of various algae is closely simulated, and in

other plants, such as the frondose liverworts, the same thalloid and

branched form of body is again met with. Some of the much dissected

_Aphlebia_ leaves of ferns (e.g. _Rhacophyllum_ species) bear a

striking resemblance to fossil algae; and numerous other examples might

be quoted. In palaeobotanical literature we find a host of names, such

as _Chondrites_, _Fucoides_[224], _Caulerpites_ and others applied to

indefinite and indistinct surface markings which happen to resemble in

shape certain of the better known genera of recent seaweeds.

The close parallelism in outward form displayed by different genera

and families of algae is in itself sufficient argument against the use

of recent generic names for fossils of which the algal nature is often

more than doubtful. Were external form to be accepted as a trustworthy

guide, in the absence of internal structure and reproductive organs,

such a genus as _Caulerpa_[225] would afford material for numerous

generic designations. A comparison of the different species of this

Siphoneous green alga brings out very clearly the exceedingly protean

nature of this interesting genus, and serves as one instance among

many of the small taxonomic value which can be attached to external

configuration. _Caulerpa pusilla_ Mart. and Her., _C. taxifolia_

(Vahl.), _C. plumaris_ Forsk., _C. abies-marina_ J. Ag., _C.

ericifolia_ (Turn.), _C. hypnoides_ (R. Br.), _C. cactoides_ (Turn.),

_C. scalpelli formis_ (R. Br.), and others clearly illustrate the

almost endless variety of form exhibited by the species of a single

genus of algae. We constantly find in the several classes of plants

a repetition of the same form either in the whole or in the separate

members of the vegetative body, and but a slight acquaintance with

plant types should lead us to use the test of external resemblance

with the greatest possible caution. To emphasize this danger may seem

merely the needless reiteration of a self-evident fact, but there is,

perhaps, no source of error which has been more responsible for the

creation of numerous worthless species among fossil plants.

[Illustration: FIG. 30. 1. Rill-mark (after Williamson). 2. Trail made

by a seaweed dragged along a soft plaster of Paris surface (after

Nathorst). 3. Tracks made by _Goniada maculata_, a Polychaet (after

Nathorst). 4. Burrow of an insect. 4_a_. Section of the gallery

(after Zeiller).]

There is, however, another category of impressions and casts of common

occurrence in sedimentary rocks which requires a brief notice. Very

many of the fossil algae described in text-books and palaeobotanical

memoirs have been shown to be of animal origin, and to be merely

the casts of tracks and burrows. A few examples will best serve to

illustrate the identity of many of the fossils referred to algae with

animal trails and with impressions produced by inorganic agency.

Dr Nathorst of Stockholm has done more than any other worker to

demonstrate the true nature of many of the species of _Chondrites_,

_Cruziana_, _Spirophyton_, _Eophyton_, and numerous other genera. In

1867 there were discovered in certain Cambrian beds of Vestrogothia,

long convex and furrowed structures in sandstone rocks which were

described as the remains of some comparatively highly organised plant,

and described under the generic name _Eophyton_[226]. By many authors

these fossils have been referred to algae, but Nathorst has shown that

the frond of an alga trailed along the surface of soft plaster of Paris

produces a finely furrowed groove (fig. 30, 2) which would afford a

cast similar to that of _Eophyton_. The same author has also adduced

good reasons for believing that the Eophytons of Cambrian rocks may

represent the trails made by the tentacles of a _Medusa_ having a habit

similar to that of _Polydonia frondosa_ Ag. Impressions of _Medusae_

have been described by Nathorst from the beds in which _Eophyton_

occurs; and the specimens in the Stockholm Museum afford a remarkable

instance of the rare preservation of a soft-bodied organism[227]. By

allowing various animals to crawl over a soft-prepared surface it is

possible to obtain moulds and casts which suggest in a striking manner

the branched thallus of an alga. The tracks of the Polychaet, _Goniada

maculata_ Örstd.[228], one of the Glyceridae, are always branched and

very algal-like in form (fig. 30, 3). Many of the so-called fossil

algae are undoubtedly mere tracks or trails of this type. In the

fossil-plant gallery of the British Museum there are several specimens

of small branched casts, clearly marked as whitish fossils on a dark

grey rock of Lower Eocene age from Bognor; these were described by

Mantell and Brongniart[229] as an alga, but there is little doubt of

their being of the same category as the track shown in fig. 30, 3.

[Sidenote: FOSSILS SIMULATING ALGAE.]

The well-known half-relief casts met with in Cambrian, Silurian and

Carboniferous rocks, and known as _Cruziana_ or Bilobites, are probably

casts of the tracks of Crustaceans. The impression left by a King-Crab

(_Limulus_) as it walks over a soft surface affords an example of this

form of cast. It has been suggested that some of the Bilobites may

be the casts of an organism like _Balanoglossus_[230], a worm-like

animal supposed by some to have vertebrate affinities. The resemblance

between some of the lower Palaeozoic Bilobites and the external

features of a _Balanoglossus_ is very striking, and such a comparison

is worth considering in view of the fact that soft-bodied animals have

occasionally left distinct impressions on ancient sediments.

The literature on the subject of fossil algae _versus_ inorganic and

animal markings is too extensive and too wearisome to consider in

a short summary; the student will find a sufficient amount of such

controversial writing—with references to more—in the works quoted

below[231].

In the Stockholm Museum of Palaeobotany there is an exceedingly

interesting collection of plaster casts obtained by Dr Nathorst

in his experiments on the manufacture of fossil ‘algae,’ which

afford convincing proof of the value and correctness of his general

conclusions.

The pressure of the hand on a soft moist surface produces a raised

pattern like a branched and delicate thallus. The well-known _Oldhamia

antiqua_ Forbes and _Oldhamia radiata_ Forbes[232], from the Cambrian

rocks of Ireland may, in part at least, owe their origin to mechanical

causes, and we have no sufficient evidence for including them among

the select class of true fossil algae. Sollas[233] has shown that the

structure known as _Oldhamia radiata_ is not merely superficial but

that it extends across the cleavage-planes. _Oldhamia_ is recorded from

Lower Palaeozoic rocks in the Pyrenees[234] by Barrois, who agrees with

Salter, Göppert and others in classing the fossil among the algae. The

photograph accompanying Barrois’ description does not, however, add

further evidence in favour of accepting _Oldhamia_ as a genus of fossil

algae.

The burrows made by _Gryllotalpa vulgaris_ Latr., the Mole-cricket,

have been shown by Zeiller to bear a close resemblance to a branch of a

conifer in half-relief (fig. 30, 4), or to such a supposed algal genus

as _Phymatoderma_[235].

In fig. 30, 1, we have what might well be described as a fossil

alga. This is merely a cast of a miniature river-system such as one

frequently sees cut out by the small rills of water flowing over a

gently-sloping sandy beach. A cast figured and described by Newberry

as an alga, _Dendrophycus triassicus_[236], from the Trias of the

Connecticut Valley, is practically identical with the rill-marks

shown in fig. 30, 1. The cracks produced in drying and contracting

sediment may form moulds in which casts are subsequently produced by

the deposition of an overlying layer of sand, and such casts have

been erroneously referred to algal impressions[237]. Dawson[238] has

figured two good examples of Carboniferous rill-marks from Nova Scotia

in his paper on Palaeozoic burrows and tracks of invertebrate animals.

[Sidenote: RECOGNITION OF FOSSIL ALGAE.]

[Illustration: FIG. 31. _Chondrites verisimilis_ Salt. Wenlock

limestone, Dudley. From a specimen in the British Museum (V. 2550).

Slightly reduced.]

The specimen represented in fig. 31 affords an example of a fairly

well-known fossil from the Wenlock limestone, originally described by

Salter as _Chondrites verisimilis_ Salt, from Dudley[239]. He regarded

it as an alga, and the graphitic impression agrees closely in form

with the thallus of some small seaweeds. A closer examination of the

fossil reveals a curious and characteristic irregular wrinkling on the

graphite surface, which suggests an organism of more chitinous and

firmer material than that of an alga.

A similar and probably an identical fossil is described and figured

by Lapworth[240] in an appendix to a paper by Walter Keeping on the

geology of Central Wales, under the name of _Odontocaulis Keepingi_

Lap. and regarded as a dendroid graptolite. In any case we have no

satisfactory grounds for including these fossils in the plant-kingdom.

How then are we to recognise the traces of ancient algae? There is no

golden rule, and we must admit the difficulty of separating real fossil

algae from markings made by animal or mechanical agency. The presence

of a carbonaceous film is occasionally a help, but its occurrence is

no sure test of plant origin, nor is its absence a fatal objection

to an organic origin. While being fully alive to the small value of

external resemblance, and to the numerous agents which have been shown

to be capable of producing appearances indistinguishable from plant

impressions, we must not go too far in a purely negative direction.

An important contribution to the subject of fossil algae has lately

appeared by Prof. Rothpletz[241]. He deals more particularly with

the much discussed Flysch[242] Fucoids of Tertiary age, and while

refusing to accept certain examples as fossil algae, he brings forward

weighty arguments in favour of including several other forms among

the algae. He is of opinion that most of the main divisions of the

algae are represented among the Flysch Fucoids, but considers that the

Phaeophyceae are the most numerous.

Rothpletz’s work is chiefly interesting as illustrating the application

of microscopic examination and chemical analysis to the determination

of fossil algae. Although he makes out a good case in favour of

restoring many of the Tertiary fossils to the plant kingdom, the

material at his disposal does not admit of satisfactory botanical

diagnosis.

No doubt some of the fossils from the Silurian and Cambrian rocks are

true algae, and Nathorst has pointed out that such a species as Hall’s

_Sphenothallus angustifolius_[243] may well be an alga. Additional

examples might be quoted from Bornemann and other writers, but in view

of the attempts which are sometimes made to trace the development of

more recent plants to more than doubtful Lower Palaeozoic Algae, one

must agree with Nathorst’s opinion,—“Je crois que l’on rend un bien

mauvais service à la théorie de l’évolution, en essayant de baser

l’arbre généalogique des algues fossiles sur des corps aussi douteux

que les Bilobites, Crossochorda, Eophyton, etc.[244]”

There are many carbonaceous impressions on rocks of different ages

which it is reasonable to refer to algal origin, and although such

are of little or no botanical value, it may be a convenience to

refer to them under a definite term. The comprehensive generic name

_Algites_[245] has been suggested as a convenient designation for

impressions or casts which are probably those of algae.

[Sidenote: SUPPOSED FOSSIL ALGAE.]

Some of the fossils described by Mr Kidston from British Carboniferous

rocks as probably algae present an undoubted algal appearance, and

might be placed in the genus _Algites_; but in some cases—e.g.

_Chondrites plumosa_[246] Kidst. from the Calciferous Sandstone of

Eskdale, one feels much more doubtful; in this particular instance the

impressions suggest the fine roots of a water-plant.

The statement is occasionally made that the numerous fossil algae

and the absence of higher plants in the older strata justify the

description of the oldest rocks as belonging to the ‘age of algae.’

Such an assertion rests on an unsound basis, and is rather the

expression of what might be expected than what has been proved to

be the case. The oldest plants with which we are at all closely

acquainted are of such a type as to forcibly suggest that in the lowest

fossiliferous rocks we are still very far from the sediments of that

age which witnessed the dawn of plant life.

Many of the obscure markings on rock surfaces which have been referred

to existing genera of algae or described as new genera, are much

too doubtful to be included even under such a comprehensive name as

_Algites_. Space does not admit of further reference to determinations

of this type which abound in palaeontological literature.

It would be very difficult to produce satisfactory evidence for the

algal nature of many of the supposed fossil algae from Cambrian

rocks[247]; there has been a special tendency to recognise algal

remains in the oldest fossiliferous strata, due in part no doubt to the

fallacy that in that period nothing higher than Thallophytes is likely

to have existed. The so-called _Phycodes_ referred to by Credner[248]

as characteristic of the Cambrian rocks of the Fichtelgebirge

(“Phycoden-Schiefer”) is probably of inorganic origin, and comparable

to the genus _Vexillum_ of Saporta[249] and other writers, which

Solms-Laubach has described as being formed every day in the soft mud

of our ponds where local currents are checked by branches and other

obstacles[250]. There are several good specimens of _Phycodes_ in the

Bergakademie of Berlin and in the Leipzig Museum which, I believe,

clearly demonstrate the absence of all satisfactory evidence of an

algal origin.

We may next pass to a short description of a few representative types

of algae, which may reasonably be classed under definite families, and

accepted as evidence possessing some botanical value.

A. DIATOMACEAE (BACILLARIACEAE).

This family occupies a somewhat isolated position among the algae, and

is best considered as a distinct subdivision rather than as a family of

the Phaeophyceae or Brown algae, with which it possesses as a common

characteristic a brown-colouring matter.

Single-celled plants consisting of a simple protoplasmic body

containing a nucleus and brown colouring matter (diatomin) associated

with the chlorophyll. The cell-wall is in the form of two halves,

known as _valves_, which fit into one another like the two portions of

a pill-box. The cell-wall contains a large amount of silica, and the

siliceous cases of the diatoms are commonly spoken of as the valves of

the individual, or the _frustules_. Diatoms exhibit a characteristic

creeping movement, and are reproduced by division, also by the

development of spores in various forms[251].

The recent members of the family have an exceedingly wide distribution,

occurring both in freshwater and in the sea. Owing to the lightness

of the frustules, they are frequently carried along in the air, and

atmospheric dust falling on ships at sea has been found to contain

large numbers of diatoms[252]. The siliceous valves are abundant

in guano deposits, and they have been found also in association

with volcanic material. Diatomaceous deposits are now being formed

in the Yellowstone Park district; “they cover many square miles in

the vicinity of active or extinct hot spring vents of the park, and

are often three feet, four feet, and sometimes five to six feet

thick[253].” The gradual accumulation of the siliceous tests on the

floor of a fresh-water lake results in the formation of a sediment

consisting in part of pure silica. Such deposits, often spoken of as

_kieselguhr_ or _diatomite_, and used as a polishing material, occur

in many parts of Britain, marking the sites of dried-up pools or lakes.

At the northern end of the island of Skye there occurs an unusually

pure deposit of diatomite overlain by peat and turf, and extending over

an area of fifty-eight square miles. Many of the individuals in this

deposit were in all probability carried into the lake by running water,

while others lived in the lake and after death their tests contributed

to the siliceous deposit[254]. The late Dr Ehrenberg published numerous

papers on diatomaceous deposits in different parts of the world, and

in his great work, _Zur Mikrogeologie_[255], he gave numerous and

beautifully executed illustrations of such siliceous accumulations. In

many of the samples he figures one sees fragments of plant tissues,

spores of conifers and ferns, associated with the diatom tests. The

occurrence of the pollen grains of coniferous trees in lacustrine and

marine deposits is not surprising in view of their abundance in Lake

Constance and other lakes. It is stated that the pollen of conifers in

the Norwegian fiords plays an important part in the nourishment of the

Rhizopod _Saccamina_[256].

[Sidenote: DIATOMACEOUS OOZE.]

In the waters of the ocean diatoms are of frequent occurrence, and

very widely distributed. Sir Joseph Hooker records the existence of

masses of diatomaceous ooze over a wide area in Antarctic regions[257].

Along the shores of the Victoria Barrier, a perpendicular wall of ice,

between one and two hundred feet above sea-level, the soundings were

found to be invariably charged with diatom remains, and from the base

of the ice-wall there appeared to be in process of formation a bank

of these tests stretching north for a distance of 200 miles. The more

extended researches conducted during the cruise of the Challenger

have clearly proved the enormous accumulations of diatoms now being

formed on the ocean-bed[258]. South of latitude 45° S. there is now

being built up a vast deposit which may be eventually upraised as a

fairly pure siliceous rock. From extreme northern latitudes Nansen has

recently recorded the occurrence of these lowly organised plants.

He writes,—“I found a whole world of diatoms and other microscopical

organisms, both vegetable and animal, living in the fresh-water pools

on the Polar drift-ice, and constantly travelling from Siberia to the

east coast of Greenland[259].” In warmer latitudes diatoms abound in

the surface waters, but there they are associated with numerous other

forms of the Plankton vegetation. The waters of the Amazon carry with

them into the sea large numbers of fresh-water forms, which are floated

out to sea and finally added to the rock-building material which is

constantly accumulating on the ocean floor[260]. No definite results

have so far been obtained as to the geographical and bathymetrical

distribution of marine diatoms.

The enormous number of recent species precludes any attempt to give a

description of the better-known forms. It is more important for us to

realize how common and widely distributed are the living genera. The

hard and almost indestructible valves have been frequently found in a

fossil condition, often forming thick and extensive masses of siliceous

rock. From diatom-beds now forming in lakes and on the ocean-bed we

pass to deposits such as those in Skye and elsewhere, which mark the

site of recently dried-up sheets of water, and so to older rocks of

Tertiary age formed under similar conditions. Among the many examples

of diatomaceous deposits of Tertiary and Cretaceous age mention

should be made of those of Berlin, Königsberg, Bilin in Bohemia, and

Richmond in Virginia. The diatoms in the beds of Berlin are regarded

as fresh-water, and those of Richmond as marine. It has been pointed

out by Pfitzer that it is a comparatively easy matter to distinguish

between fresh-water and marine forms of diatoms. The diatomaceous rocks

of Bilin are known as polishing slates; they attain a thickness of 50

feet. In these, as in many other cases, the deposit has become cemented

together as a hard flinty or glassy rock, in which the cementing

material was formed by the solution of some of the diatom tests[261].

In many cases in which calcareous and siliceous rocks reveal no direct

evidence of organic origin it is probable that they were originally

formed by the accumulation of plants of which the structure has been

completely obliterated by secondary causes. The genus _Gallionella_

plays an important part in the composition of the Bilin beds.

Occasionally impressions of leaves and other organic remains are found

associated with the diatoms in the siliceous rocks. In the British

Museum (Botanical department) a large block of white powdery rock is

exhibited as an example of a diatomaceous deposit of Tertiary age from

Australia. It is described as being largely made up of the tests of

fresh-water diatoms, such as _Navicula_, _Gomphonema_, _Cymbella_,

_Synedra_, and others.

[Sidenote: FOSSIL DIATOMS.]

The abundance of Diatoms in Cretaceous rocks of the Paris basin has

recently been recorded by Cayeux[262]; it would seem that these algae

had already assumed an important rôle as rock-builders in pre-Tertiary

times. Cayeux points out that the silica of these Cretaceous

diatomaceous frustules has often been replaced by carbonate of calcium.

In addition to the occurrence of Diatoms in the various diatomaceous

deposits, their siliceous tests may occasionally be recognised in

argillaceous or other sediments. Shrubsole and Kitton[263] have

described several species of Diatoms from the London Clay of Lower

Eocene age. In many localities in the London basin the clay obtained

from well-sinkings presented the appearance of being dusted with

sulphur-like particles of a dark bronze or golden colour which

glistened in the sunlight. These yellow bodies have been found to

be diatomaceous frustules in which the silica has been replaced by

iron pyrites. The genus _Coscinodiscus_ is one of the commonest forms

recorded from the London Clay[264].

Without further considering individual examples of diatomaceous rocks

we may briefly notice the general facts of the geological history of

the family. As Ehrenberg pointed out several years ago, the Tertiary

and Cretaceous species of diatoms show a very marked resemblance to

living forms. In many cases the species are identical, and the fossil

deposits as a whole seem to differ in no special respect from those now

being built up.

With the exception of two species of Liassic Diatoms, no trustworthy

examples of the Diatomaceae have been found below the Cretaceous

series. The oldest known Diatoms were discovered by Rothpletz[265]

among the fibres of an Upper Lias sponge from Boll in Württemberg.

They occur as small thimble-shaped siliceous tests with coccoliths

and foraminifera in the horny skeleton of _Phymatoderma_, a genus

formerly regarded as an alga. Rothpletz describes two species which he

includes in the genus _Pyxidicula_, _P. bollensis_ and _P. liasica_.

This generic name of Ehrenberg is used by Schütt[266] as a subgenus of

_Stephanopyxis_.

Seeing how great a resemblance there is between the recent and

Cretaceous species, and how many examples there are of Tertiary diatom

deposits, it is not a little surprising that the past history of these

plants has not been traced to earlier periods. In 1876 Castracane[267],

an Italian diatomist, gave an account of certain species of diatoms

said to have been found in a block of coal from Liverpool obtained

from the English Coal-Measures. The species were found to be identical

with recent forms. It is generally agreed that these specimens

cannot have been from the coal itself, but that they must have been

living forms which had come to be associated with the coal. The late

Prof. Williamson spent many years examining thin sections and other

preparations of coal from various parts of the world, but he never

found a trace of any fossil diatom. There is no apparent reason

why diatoms should not be found in Pre-Cretaceous rocks, and the

microscopic investigation of old sediments may well lead to their

discovery. Prof. Bertrand of Lille, who has devoted himself for some

time past to a detailed microscopical examination of coal, informs

me that he has so far failed to discover any trace of Palaeozoic

diatomaceous tests.

[Sidenote: BACTRYLLIUM.]

The genus _Bactryllium_ is often quoted in text-books as a probable

example of a Triassic diatom. It was first described by Heer[268] from

the Trias of Switzerland and North Italy, also from the neighbourhood

of Heidelberg, and regarded as an extinct member of the Diatomaceae.

Heer defined the genus as follows:

“Small bodies, with parallel sides, rounded at either end, the

surface traversed by one or two longitudinal grooves.”

(fig. 32, C.) Several species have been figured by Heer from beds of

Muschelkalk, Keuper and Rhaetic age. He describes the wall as thick and

firm (fig. 32, C. ii.) and probably composed of silica, with a hollow

interior. The specimen shown in fig. 32, C. was found in the Rhaetic

beds, and named by Heer _Bactryllium deplanatum_; it has a length

of 4·5 mm.; the surface is transversely striated and traversed by a

single longitudinal groove. Stefani[269] has given reasons in favour

of removing _Bactryllium_ from the plant to the animal kingdom; he

points out that the specimens are too large for diatoms, and moreover

that they are asymmetrical in form and possessed a calcareous and not

a siliceous shell. He would place the fossil among the Pteropods,

comparing it with such genera as _Cuvierina_ and _Hyalaea_. In view

of Stefani’s opinion we cannot attach any importance to this supposed

diatom, especially as it has generally been regarded as at best but an

unsatisfactory genus.

[Illustration: FIG. 32. _A_, _Lithothamnion mamillosum_ Gümb. (i) In

section, (ii) surface view [after Gümbel. (i) × 320, (ii) nat.

size]. _B_, _Sycidium melo_ Sandb. (i) Surface view, (ii)

transverse section (after Deecke). _C_, _Bactryllium deplanatum_

Heer. (i) Surface view, (ii) section, showing the thick wall and

hollow interior (after Heer). _D_, Calcareous pebble from a lake in

Michigan. Rather less than nat. size (after Murray).]

B. CHLOROPHYCEAE (GREEN ALGAE).

Thallus unseptate, having the form of a vesicle or a variously branched

coenocyte, which may or may not be encrusted with carbonate of lime, or

of filaments composed of cells containing a single nucleus, or of cells

in which more than one nucleus occurs; in other instances consisting

of a plate of cells or a cell-mass. Asexual reproduction by zoospores

and other reproductive cells; sexual reproduction by means of the

conjugation of similar gametes or by the fertilisation of a typical

egg-cell by a motile spermatozoid.

This family of algae is represented at the present day by numerous

and widely distributed marine and fresh-water genera, as well as by

genera growing in moist air or as endophytes in the tissues of higher

plants[270].

Seeing how very few fossil forms have been described which have any

claim to inclusion in this subdivision of the Algae, it is unnecessary

to enumerate or define the various families of the Chlorophyceae. It

is true that many species have been figured as examples of different

genera of green algae, but few of these possess any scientific value.

There is, however, one division of the Chlorophyceae, the Siphoneae,

which must be treated at some length on account of its importance from

a palaeobotanical and geological point of view.

_a._ SIPHONEAE.

Thallus consisting of simple or branched cells very rarely divided

by septa, and containing many nuclei. In certain genera the branches

form a pseudoparenchymatous tissue by their repeated branching, and

as a result of the intimate felting together of the branched cells.

Reproduction is effected either by the conjugation of similar gametes

or by the fertilisation of an egg-cell.

_Vaucheria_ and _Botrydium_ are two well-known British genera of

this order, but most of the recent representatives live in tropical

and subtropical seas. The most striking characteristic feature of

this division of the Chlorophyceae is the fact that the thallus of a

siphoneous alga consists of an unseptate coenocyte; the plant may be

extremely small and simple, or it may reach a length of several inches,

but in all cases the body does not consist of more than one cell or

coenocyte.

From a palaeontological standpoint the Siphoneae are of exceptional

interest. It is impossible to do more than refer to a few of the living

and fossil genera. There are numerous fossil representatives already

known, and there can be little doubt that further research would be

productive of valuable results.

As examples of the order, a few genera may be described belonging to

the three families Caulerpaceae, Codiaceae, and Dasycladaceae.

α. Caulerpaceae.

Thallus unseptate, showing an extraordinary variation in the external

differentiation of the plant-body. Reproduction is effected by means of

detached portions of the parent plant.

The genus _Caulerpa_, represented by a few species in the Mediterranean

and by many tropical forms, has already been alluded to as a striking

example of a plant which appears under a great many different

forms[271]. As a recent writer has said, “Nature seems to have shown in

this genus the utmost possibilities of the siphoneous thallus[272],”

Fragments of coniferous twigs, the tracks and burrows of various

animals and other objects have been described by several authors as

fossil species of _Caulerpa_. As an illustration of the identification

of a very doubtful fossil as a species of _Caulerpites_, reference

may be made to such a form as _C. cactoides_ Göpp.[273] from Silurian

and Cambrian rocks. There are several examples of this fossil in the

Brussels Museum which probably owe their origin to some burrowing

animal, and may be compared with Zeiller’s figures of the tunnels made

by the mole-cricket (fig. 30, 4)[274].

Mr Murray, of the British Museum, has recently described what he

regards as a trustworthy example of a fossil _Caulerpa_ from the

Kimeridge Clay near Weymouth[275]. Specimens of the fossil were first

figured in a book on the geology of the Dorset coast as casts of an

equisetaceous plant[276].

To this fossil Murray has assigned the name _Caulerpa Carruthersi_,

and given to it a scientific diagnosis. The best specimens have the

form of a slender central axis, giving off at fairly regular intervals

whorls of short and somewhat clavate branches; they bear a superficial

resemblance to such a recent species as _Caulerpa cactoides_ Ag. An

examination of several examples of this fossil leads me to express

the opinion that there is not sufficient reason for assigning to them

the name of a recent genus of algae[277]. To use the generic name of

a recent plant without following the common custom of adding on the

termination “ites” (i.e. _Caulerpites_) is as a general rule to be

avoided in dealing with fossil forms; and there are, I believe, no

satisfactory grounds for referring to these fossils as trustworthy

examples of a Mesozoic alga.

In the present case I am disposed to regard the _Caulerpa_-like casts

as of animal rather than plant origin. The clavate branches have

the form of very deep moulds in the hard brown rock which have been

filled in with blue mud. It is hardly conceivable that the branches

of a soft watery plant such as _Caulerpa_ could leave more than a

faint impression on an old sea-floor. The specimens occur in different

positions in the matrix of the rock and they are not confined to

the lines of bedding; in none of the examples is there any trace of

carbonaceous matter in association with the deep moulds. On the whole,

then, this Kimeridge fossil cannot, I believe, be accepted as an

authentic example of a Mesozoic _Caulerpa_.

It is not improbable that some of the supposed fossil algae may be

casts of egg-cases or spawn-clusters of animals. In Ellis’ Natural

History of the Corallines[278] there is a drawing representing a number

of disc-like ovaries attached to a tough ligament, and referred to the

mollusc _Buccinum_, which bears a certain resemblance to the Weymouth

fossil. A similar body is figured by Fuchs[279] in an important memoir

on supposed fossil algae.

It is not suggested that the _Caulerpa Carruthersi_ of Murray should be

regarded as the cast of some molluscan egg-case attached to a slender

axis, but it is important to bear in mind the possibility of matching

such extremely doubtful fossils with other organic bodies than the

thallus of a _Caulerpa_. In an example of an egg-case in the Cambridge

Zoological Museum, referred to a species of _Pyrula_, there is a hard,

long and slender axis, bearing a series of semicircular chambers

divided into radial compartments. The whole is hard and horny and might

well be preserved as a fossil.

β. Codiaceae.

The members of this Order present a considerable diversity of form as

regards the shape of the plant-body; the thallus of some species is

encrusted with carbonate of lime. The order is widely distributed in

tropical and temperate seas.

Among the recent genera _Penicillus_ and _Codium_ may be chosen as

important types from the point of view of fossil representatives.

_Codium._

The thallus of _Codium_ consists of a spongy mass of tubular

cell-branches which are differentiated into two fairly distinct

regions, an outer peripheral layer in which the branches have long

club-shaped terminations, and an inner region consisting of loosely

interwoven filaments.

_Codium Bursa_ L. and _C. tomentosum_ Huds. are two well-known British

species, the former presents the appearance of a spongy ball of

cells, and in the latter the thallus is divided up into dichotomously

forked branches[280]. In this genus the thallus is not encrusted with

carbonate of lime, at least in recent species.

_Sphaerocodium._ Fig. 37, D.

Rothpletz[281] instituted this genus for certain small spherical

or oval bodies varying from 1 mm. to 2 cm. in diameter, which have

been found on crinoid stems or shell fragments of Triassic age. Each

spherical body consists of dichotomously branched single-celled

filaments, between 50 and 100µ in breadth, and from 300–500µ in height.

The tubular cavities occasionally swell out into spherical spaces which

are regarded by Rothpletz as sporangia.

There is not sufficient evidence that _Sphaerocodium Bornemanni_ Roth.

has been correctly referred to the Codiaceae. The sporangia-like

swellings described by the author of the species are not by any means

conclusive as characters of important taxonomic value. Figure 37, D,

illustrates the general structure of the fossil as seen in a transverse

section of one of the calcareous grains.

Like _Girvanella_, which has been referred by some writers to the

Siphoneae, _Sphaerocodium_ occurs in the form of oolitic grains. In

the Triassic Raibler and St Cassian beds of the Tyrol, as well as in

rocks of Rhaetic age in the Eastern Alps, it makes up large masses

of limestone. Rothpletz compares the structure of this genus with

that of the recent alga _Codium adhaerens_ Ag., but it is wiser to

regard such tubular structures as _Girvanella_, _Siphonema_[282] and

_Sphaerocodium_ as closely allied organisms, which are probably algae,

but too imperfectly known to be referred to any particular family.

_Penicillus._

The recent genus _Penicillus_ is one of those algae formerly included

among animals. Fig. 33, O, has been copied from a drawing of a species

of _Penicillus_ given by Lamouroux[283] under the generic name of

_Nesea_ in his treatise on the genera of Polyps published in 1821. He

describes the genus as a brush-like Polyp with a simple stem.

The thallus consists of a stout stem terminating in a brush-like tuft

of fine dichotomously-branched filaments. The apical branches are

divided by regular constrictions into short oval or rod-like segments

which may be encrusted with carbonate of lime. A few of the segments

from the terminal tuft of a recent _Penicillus_ are shown in fig. 35,

E. Each of these calcareous segments has the form of an oval shell

perforated at each end, and the wall is pierced by numerous fine

canals. _Penicillus_ is represented by about 10 recent species, which

with one exception live in tropical seas.

The recognition of _Penicillus_, or a very similar type, in a fossil

condition is due to Munier-Chalmas[284]. This keen observer has

rendered great service to palaeobotany by directing attention to the

calcareous algae in the Paris basin beds, and by proving that many of

the fossils from these Tertiary deposits have been erroneously included

by previous writers among the Foraminifera[285]. It is greatly to be

desired that Prof. Munier-Chalmas may soon publish a monograph on the

fossil Siphoneous forms of which he possesses a unique collection.

_Ovulites._ Figs. 33, K, L, and 35, F.

In his Natural History of Invertebrate Animals, Lamarck described

some small oval bodies from the Calcaire Grossier (Eocene) of the

Paris basin under the name of _Ovulites_. He defined them as

follows:—“Polypier pierreux, libre, ovuliforme ou cylindracé, creux

intérieurement, souvent percé aux deux bouts. Pores très petits,

régulièrement disposés à la surface[286].”

[Illustration: FIG. 33. A and B, _Cymopolia barbata_ (L.); A,

transverse section of the calcareous cylinder. B, verticillate

branches and sporangium after removal of the calcareous matrix (A

and B after Munier-Chalmas). C and D, _Acicularia Andrussowi_ Solms

(C, after Andrussowi; D, after Solms). E, _Acicularia Miocenica_

Reuss; section of a spicula (after Reuss). F and G, _Acicularia

sp._ (after Carpenter), F × 40; G × 20. H, _Acicularia Schencki_

(Möb.) (after Solms). I, _Acetabularia Mediterranea_ Lamx.; section

of the cap (after Falkenberg). K and L, _Ovulites margaritula_

(Lam.) (after Munier-Chalmas); K slightly enlarged; L, a piece of

the thallus more highly magnified. M, _Cymopolia barbata_ (L.)

(after Ellis, nat. size). N, _C. barbata_ (L.); the surface of the

thallus; magnified. O, _Penicillus pyramidalis_ (Lamx.) (after

Lamouroux, nat. size).]

The specimens are referred to two species, _Ovulites margaritula_ and

_O. elongata_.

By some subsequent writers[287] these calcareous fossils, like

miniature birds’ eggs with a hole at either end, were included among

the Zoophytes. Carpenter and others afterwards referred _Ovulites_ to

the Foraminifera, and compared the genus with Lagena[288]. The single

specimens of _Ovulites_ have a length of 2–6 mm. At each end there is

usually a fairly large and somewhat irregular hole (fig. 35, F), and

in some rarer cases there may be two apertures at the broader end of

an Ovulite. A good example of _Ovulites margaritula_ with two pores at

the broader end is figured by Michelin[289]. The surface of the shell

when seen under a low magnifying power appears to be covered over with

regularly arranged circular pores, which are the external openings of

fine canals (fig. 33, L).

In 1878 Munier-Chalmas expressed the opinion, which was supported by

strong evidence, that _Ovulites_ should be referred to the siphoneous

algae[290]. He regarded it as generically identical with _Penicillus_

(_Coralliodendron_, Kützing). It has already been pointed out that

in _Penicillus_ the apical tuft of filaments is partially calcareous

(fig. 33, O)[291]. The individual calcareous segments agree almost

exactly with the fossil _Ovulites_. As a rule the Ovulites occur

as separate egg- or rod-like bodies, but Munier-Chalmas informs me

that occasionally two or three have been found joined end to end in

their natural position. The terminal holes in the fossil specimens

represent the apertures left after the detachment of the calcareous

segments from the uncalcified filaments of the alga. The segments

with two holes at the broader end were no doubt situated at the base

of dichotomising branches as shown in fig. 33, K. The restoration of

_Ovulites_, shown in fig. 33, K, bears a striking resemblance to the

figure of an Australian _Penicillus_ given by Harvey in his _Phycologia

Australica_[292].

It is probable that these Eocene forms agreed closely in habit with

the recent species of _Penicillus_. The portions preserved as fossils

are segments of the filaments which probably formed a terminal brush

of fine branches supported on a stem. The retention of the original

generic name _Ovulites_ is on the whole better than the inclusion of

the fossil species in the recent genus. The Tertiary species lived in

warm seas of the Lower and Middle Eocene of England, Belgium, France

and Italy.

_Halimeda._

An example of an Eocene species of _Halimeda_ has been recorded by

Fuchs from Greifenstein under the name of _Halimeda Saportae_[293]. The

impression has the form of a branched plant consisting of wedge-shaped

or oval segments, and there is a close resemblance to the thallus of a

recent _Halimeda_, e.g. _H. gracilis_ Harv. It is not improbable that

Fuchs’ determination is correct, but without more definite evidence

than is afforded by a mere impression it is a little rash to make use

of the recent generic name.

γ. Dasycladaceae.

In this family of Siphoneae are included a number of genera represented

by species living in tropical and subtropical seas.

The thallus consists of an elongated axial cell fixed to the substratum

by basal rhizoids, and bearing whorls of lateral appendages of limited

growth which may be either simple or branched. Many of the lateral

branches bear sporangia or spores. The thallus is in many species

encrusted with carbonate of lime.

The two genera _Acetabularia_ and _Cymopolia_ may be briefly described

as recent types which are represented by trustworthy fossil forms.

[Illustration: FIG. 34. _Acetabularia mediterranea_ Lamx. From a

specimen in the Cambridge Botanical Museum (nat. size).]

_Acetabularia._ Figs. 33, I, and 34.

With the exception of _A. mediterranea_ Lamx. (fig. 34) the few living

species of this genus are confined to tropical seas.

The habit of _Acetabularia_ is well illustrated by the photograph of a

cluster of plants of _A. mediterranea_ Lamx.[294] reproduced in fig.

34. The thallus consists of a delicate stalk attached to the substratum

by a tuft of basal holdfasts, and expanded distally into a small

circular disc 10–12 mm. in diameter and more or less concave above.

This terminal cap is made up of a number of laterally fused appendages

given off from the upper part of the stalk in the form of a crowded

whorl. The whole thallus resembles a small and long-stalked calcareous

fungus. In each radially elongated compartment of the fertile cap

(fig. 33, I) there are several sporangia (_gametangia_) developed;

these eventually open and produce numerous ciliated gametes which

give rise to zygospores by conjugation. Fig. 33, I, represents the

cap of an _Acetabularia_ in radial section and surface-view; the two

radial compartments seen in section contain the elliptical gametangia;

the circular markings at the base of the figure are scars of sterile

deciduous branches.

The whole plant is unicellular, each chamber in the disc being in open

communication with the stem of the plant.

_Acicularia._ Fig. 33, C–H.

In a recent monograph on the Acetabularieae, Solms-Laubach[295] has

described a new type of these algae which is of special importance

from the point of view of the past history of the family. Möbius

described an example of _Acetabularia_ in 1889 under the name _A.

Schencki_; this species has since been placed in D’Archiac’s genus

_Acicularia_[296]. _Acicularia Schencki_[297] bears a close resemblance

as regards external form to _Acetabularia mediterranea_. In the latter

species the walls of the terminal disc compartments are calcified,

and the cavity of each of the laterally fused members contains

numerous free spores; in _Acicularia_, the cavity of each disc-ray is

occupied by a calcareous substance in the form of a spicule containing

numerous cavities in each of which is a single sporangium. A single

spicule is seen in fig. 33, H, showing the spherical pockets in which

the sporangia were originally situated. This species, _Acicularia

Schencki_, has been recorded from Martinique, Guadeloupe, Brazil, and a

few other places.

The genus _Acicularia_ was founded by D’Archiac for certain minute

calcareous spicules found in the Eocene sands (Calcaire Grossier)

of the Paris basin. D’Archiac describes one species, _Acicularia

pavantina_, which he defines as follows:—“Polypier aciculaire, élargi,

et légèrement comprimé à sa partie supérieure, qui est échancrée au

milieu. Surface couverte de petits pores simples, nombreux, disposés

irrégulièrement[298].” The same species is figured also in Michelin’s

_Iconographie Zoophytologique_, and described as an organism of

which the exact zoological position is uncertain[299]. After these

fossils had been placed in various divisions of the animal kingdom,

Carpenter[300] described several specimens as portions of foraminifera.

Finally, Munier-Chalmas removed _Acicularia_ to the plant kingdom,

and “with rare divination” placed the genus among the Acetabularieae.

The history of our knowledge of the true nature of _Acicularia_ is

of unusual interest. Some of the specimens of this genus figured in

Carpenter’s monograph have the form of imperfect long and narrow bodies

tapering to a point at one end and broad at the other (fig. 33, F and

G); they are joined together laterally and pitted with numerous small

cavities. From the resemblance of such specimens to a fragment of the

terminal fertile disc of the recent Acetabularias, Munier-Chalmas

referred the fossils to this type of algae. In the living species

which were then known the radiating chambers of the disc contained

loose sporangia, without any calcareous matrix filling the cavity of

the chambers. In the fossil Acicularias, on the other hand, the manner

of preservation of the pitted calcareous spicules pointed to the

occurrence of sporangia embedded in cavities in a calcareous matrix.

Subsequent to Munier-Chalmas’ somewhat daring conclusions as to the

relation of _Acicularia_ to _Acetabularia_, Solms-Laubach found that

the species originally described by Möbius as _Acetabularia Schencki_

from Guadeloupe presented exactly those characters in which the fossil

specimens differ from _Acetabularia_. The genus _Acicularia_ formerly

restricted to fossil species is now applied also to this single living

species _Acicularia Schencki_.

The genus is thus defined by Solms-Laubach:—

“Discus fertilis terminalis e radiis inter se conjunctis formatus,

coronis et inferiore et superiore praeditis, sporae massa mucosa

calce incrustata coalitae, pro radio spiculam solidam cuneatam

formantes[301].”

As Solms-Laubach points out in his recent monograph, Munier-Chalmas’

conjecture, “which had little to support it in the fossil material, has

been more recently proved true in the most brilliant fashion by the

discovery of a living species of this genus.”

• • • • •

1. _Acicularia Andrussowi_ Solms[302]. Fig. 33, C and D. This species

was first described by Andrussow[303] as _Acetabularia miocenica_ from

the Crimea. It occurs in Miocene rocks south of Sevastopol, and, with

_Ostrea_ and _Pecten_, forms masses of white limestone.

In each sporangial ray of the disc the cavity contains a calcareous

spicula bearing spore cavities in four rows. “Round each spore-cavity

there is a circular zone which stands out, when viewed in reflected

light, through its white colour against the central mass of the

spicule, though a sharp contour is not visible[304].” Fig. 33, C, is

taken from a somewhat diagrammatic sketch by Andrussow; it shows ten of

the fertile rays of the disc. The thick walls of the chambers are seen

in the two lowest rays, and in the next two rays the spore-cavities

are represented. A more accurate drawing, from Solms-Laubach’s memoir,

is reproduced in fig. 33, D. The calcareous spicule with numerous

spore-cavities shown in fig. 33, H, is from a fertile ray of the recent

species _Acicularia Schencki_. This corresponds to the spore-containing

calcareous matrix in each ray of the disc of _Acicularia Andrussowi_

Solms. The spicule copied in fig. 33, F from one of Carpenter’s

drawings[305] of an Eocene specimen bears the closest resemblance

to the recent spicule of fig. 33, H, and emphasizes the very close

relationship between the fossil forms and the single rare tropical

species.

2. _Acicularia miocenica_ Reuss. Another Tertiary species has been

described under this name by Reuss[306] from the Miocene of the Vienna

district, from the Leithakalk of Moravia and elsewhere. It agrees very

closely with the recent species _A. Schencki_. A section of one of

the spicules of this species is shown in fig. 33, E; the dark patches

represent the pockets in the calcareous spicule which were originally

occupied by sporangia and spores.

_Cymopolia_. Fig. 33, A, B, M and N.

The genus _Cymopolia_ is at present represented by two species, _C.

barbata_ (L.) and _C. mexicana_, Ag., living in the Gulf of Mexico and

off the Canary Islands.

_Cymopolia_ and _Acetabularia_, with several other calcareous algae,

are figured by Ellis and other writers as members of the animal

kingdom. Ellis speaks of the species of _Cymopolia_ which he figures as

the Rosary Bead-Coralline of Jamaica.

Fig. 33, M, has been drawn from a figure published by Ellis in his

_Natural History of the Corallines_ published in 1755[307]. The thallus

has the form of a repeatedly forked body, of which the branches are

divided into cylindrical joints thickly encrusted with carbonate of

lime, but constricted and uncalcified at the limits of each segment. A

tuft of hairs is given off from the terminal segment of each branch.

The axis of each branch of the thallus is occupied by a cylindrical

and unseptate cell which gives off crowded whorls of lateral branches.

In the lower part of fig. 33, M, the calcareous investment has been

removed, and the branches are seen as fine hair-like appendages of the

central cell. The branches given off from the constricted portions of

the axis are unbranched simple appendages, but the others terminate

in bladder-like swellings, each of which bears an apical sporangium.

The sporangia are surrounded and enclosed by the swollen tips of

four to six branches which spring from the summit of the sporangial

branch. Fig. 33, A, represents part of a transverse section through

the calcareous outer portion of a branch of _Cymopolia_; the darker

portions or cavities in the calcareous matrix were originally occupied

by the lateral branches and sporangia[308].

In Fig. 33, B, the sporangial branch with the terminal sporangium and

three of the investing branches are more clearly shown, the surrounding

calcareous investment and the thallus having been removed by the action

of an acid.

In a transverse section of a branch from which the organic matter

had been removed, and only the calcareous matrix left, one would

see a central circular cavity surrounded by a thick calcareous wall

perforated by radially disposed canals and containing globular

cavities; the canals and cavities being occupied in the living plant by

branches and sporangia respectively.

The two circular cavities shown in the figure mark the position of the

sporangia which are borne on branches with somewhat swollen tips. From

the summit the left-hand sporangial branch shown in fig. 33, A, three

of the secondary branches are represented by channels in the calcareous

matrix; the two black dots on the face of the sporangiophore being the

scars of the remaining two secondary branches.

By the lateral contact of the swollen ends of the ultimate branches

enclosing the sporangia the whole surface of the thallus, when examined

with a lens, presents a pitted appearance. Each pit or circular

depression (fig. 33, N) marks the position of the swollen tip of a

branch.

This form of thallus represents a type which is met with in several

members of the Dasycladaceae. It would carry us beyond the limits

of a short account to describe additional recent genera which throw

light on the numerous fossil species. For further information as to

the recent members of the family, the student should refer to Murray’s

_Seaweeds_[309], and for a more detailed memoir on the group to

Wille’s recent contribution to the _Pflanzenfamilien_[310] of Engler

and Prantl. Among the various special contributions to our knowledge

of the Dasycladaceae, those by Munier-Chalmas[311], Cramer[312],

Solms-Laubach[313], and Church[314], may be mentioned.

[Sidenote: PALAEOZOIC SIPHONEAE.]

The publication of a short preliminary note by Prof. Munier-Chalmas in

the _Comptes Rendus_ for 1877 was the means of calling attention to the

exceptional importance of the calcareous Siphoneae as algae possessing

an interesting past history, of which satisfactory records had been

preserved in rocks of various ages. Decaisne had pointed out in 1842

that certain marine organisms previously regarded as animals should

be transferred to the plant kingdom. Such seaweeds as _Halimeda_,

_Udotea_, _Penicillus_ and others were thus assigned to their correct

position. Many fossil algae belonging to this group continued to be

dealt with as Foraminifera until Munier-Chalmas demonstrated their true

affinities. In Gümbel’s monograph on the so-called Nullipores found in

limestone rocks, published in 1871[315], several examples of siphoneous

algae are included among the fossil Protozoa.

In recent years there have been several additions to an already

long list of fossil Siphoneae. In addition to the numerous and

well-preserved specimens, representing a large number of generic and

specific forms, which have been collected from the Eocene of the Paris

basin, there is plenty of evidence of the abundance of the members of

the Dasycladaceae in the Triassic seas. In the Triassic limestones

of the Tyrol, as well as in other regions, the calcareous bodies of

siphoneous algae have played no inconsiderable part as agents of

rock-building[316]. Genera have been recorded from Silurian and other

Palaeozoic horizons, and there is no doubt that the Verticillate

Siphoneae of to-day are the remnants of an extremely ancient family,

which in former periods was represented by a much more widely

distributed and more varied assemblage of species. There is probably

no more promising field of work in the domain of fossil algae than the

further investigation of the numerous forms included in Munier-Chalmas’

class of Siphoneae Verticillatae. A brief description of a few genera

from different geological horizons must suffice to draw attention to

the character of the data for a phylogenetic history of this group.

The fossil examples of the genus _Cymopolia_ (_Polytrypa_) were

originally described by Defrance[317] in the _Dictionnaire des Sciences

Naturelles_ as small polyps under the generic name _Polytrypa_.