THE CRETACEOUS PERIOD.
The next series of rocks in ascending order is the great and
important series of the Cretaceous Rocks, so called from the
general occurrence in the system of chalk (Lat. _creta_, chalk).
As developed in Britain and Europe generally, the following leading
subdivisions may be recognised in the Cretaceous series:--
1. Wealden, \_ Lower Cretaceous.
2. Lower Greensand or Neocomian, /
3. Gault, \
4. Upper Greensand, |_ Upper Cretaceous.
5. Chalk, |
6. Maestricht beds, /
I. _Wealden_.--The _Wealden_ formation, though of considerable
importance, is a local group, and is confined to the southeast
of England, France, and some other parts of Europe. Its name is
derived from the _Weald_, a district comprising parts of Surrey,
Sussex, and Kent, where it is largely developed. Its lower portion,
for a thickness of from 500 to 1000 feet, is arenaceous, and is
known as the Hastings Sands. Its Upper portion, for a thickness
of 150 to nearly 300 feet, is chiefly argillaceous, consisting of
clays with sandy layers, and occasionally courses of limestone.
The geological importance of the Wealden formation is very great,
as it is undoubtedly the delta of an ancient river, being composed
almost wholly of fresh-water beds, with a few brackish-water
and even marine strata, intercalated in the lower portion. Its
geographical extent, though uncertain, owing to the enormous
denudation to which it has been subjected, is nevertheless great,
since it extends from Dorsetshire to France, and occurs also in
North Germany. Still, even if it were continuous between all
these points, it would not be larger than the delta of such a
modern river as the Ganges. The river which produced the Wealden
series must have flowed from an ancient continent occupying what
is now the Atlantic Ocean; and the time occupied in the formation
of the Wealden must have been very great, though we have, of
course, no data by which we can accurately calculate its duration.
The fossils of the Wealden series are, naturally, mostly the
remains of such animals as we know at the present day as inhabiting
rivers. We have, namely, fresh-water Mussels (_Unio_), River-snails
(_Paludina_), and other fresh-water shells, with numerous little
bivalved Crustaceans, and some fishes.
II. _Lower Greensand_ (_Néocomien_ of D'Orbigny).--The Wealden
beds pass upward, often by insensible gradations, into the Lower
Greensand. The name Lower Greensand is not an appropriate one,
for green sands only occur sparingly and occasionally, and are
found in other formations. For this reason it has been proposed
to substitute for Lower Greensand the name _Neocomian_, derived
from the town of Neufchâtel--anciently called _Neocomum_--in
Switzerland. If this name were adopted, as it ought to be, the
Wealden beds would be called the Lower Neocomian.
The Lower Greensand or Neocomian of Britain has a thickness of
about 850 feet, and consists of alternations of sands, sandstones,
and clays, with occasional calcareous bands. The general colour
of the series is dark brown, sometimes red; and the sands are
occasionally green, from the presence of silicate of iron.
The fossils of the Lower Greensand are purely marine, and among
the most characteristic are the shells of _Cephalopods_.
The most remarkable point, however, about the fossils of the
Lower Cretaceous series, is their marked divergence from the
fossils of the Upper Cretaceous rocks. Of 280 species of fossils
in the Lower Cretaceous series, only 51, or about 18 per cent, pass
on into the Upper Cretaceous. This break in the life of the two
periods is accompanied by a decided physical break as well; for the
Gault is often, if not always, unconformably superimposed on the
Lower Greensand. At the same time, the Lower and Upper Cretaceous
groups form a closely-connected and inseparable series, as shown
by a comparison of their fossils with those of the underlying
Jurassic rocks and the overlying Tertiary beds. Thus, in Britain
no marine fossil is known to be common to the marine beds of
the Upper Oolites and the Lower Greensand; and of more than 500
species of fossils in the Upper Cretaceous rocks, almost everyone
died out before the formation of the lowest Tertiary strata, the
only survivors being one Brachiopod and a few _Foraminifera_.
III. _Gault_ (_Aptien_ of D'Orbigny).--The lowest member of the
Upper Cretaceous series is a stiff, dark-grey, blue, or brown
clay, often worked for brick-making, and known as the _Gault_,
from a provincial English term. It occurs chiefly in the south-east
of England, but can be traced through France to the flanks of
the Alps and Bavaria. It never exceeds 100 feet in thickness;
but it contains many fossils, usually in a state of beautiful
preservation.
IV. _Upper Greensand_ (_Albien_ of D'Orbigny; _Unterquader_ and
_Lower Plänerkalk_ of Germany).--The Gault is succeeded upward by
the _Upper Greensand_, which varies in thickness from 3 up to 100
feet, and which derives its name from the occasional occurrence
in it of green sands. These, however, are local and sometimes
wanting, and the name "Upper Greensand" is to be regarded as a
_name_ and not a description. The group consists, in Britain,
of sands and clays, sometimes with bands of calcareous grit or
siliceous limestone, and occasionally containing concretions of
phosphate of lime, which are largely worked for agricultural
purposes.
V. _White Chalk_.--The top of the Upper Greensand becomes
argillaceous, and passes up gradually into the base of the great
formation known as the true _Chalk_, divided into the three
subdivisions of the chalk-marl, white chalk without flints, and
white chalk with flints. The first of these is simply argillaceous
chalk, and passes up into a great mass of obscurely-stratified
white chalk in which there are no flints (_Turonien_ of D'Orbigny;
_Mittelquader_ of Germany). This, in turn, passes up into a great
mass of white chalk, in which the stratification is marked by
nodules of black flint arranged in layers (_Sénonien_ of D'Orbigny;
_Oberquader_ of Germany). The thickness of these three subdivisions
taken together is sometimes over 1000 feet, and their geographical
extent is very great. White Chalk, with its characteristic
appearance, may be traced from the north of Ireland to the Crimea,
a distance of about 1140 geographical miles; and, in an opposite
direction, from the south of Sweden to Bordeaux, a distance of
about 840 geographical miles.
VI. In Britain there occur no beds containing Chalk fossils, or
in any way referable to the Cretaceous period, above the true
White Chalk with flints. On the banks of the Maes, however, near
Maestricht in Holland, there occurs a series of yellowish limestones,
of about 100 feet in thickness, and undoubtedly superior to the White
Chalk. These _Maestricht beds_ (_Danien_ of D'Orbigny) contain a
remarkable series of fossils, the characters of which are partly
Cretaceous and partly Tertiary. Thus, with the characteristic
Chalk fossils, _Belemnites, Baculites_, Sea-Urchins, &c., are
numerous Univalve Molluscs, such as Cowries and Volutes, which
are otherwise exclusively Tertiary or Recent.
Holding a similar position to the Maestricht beds, and showing
a similar intermixture of Cretaceous forms with later types, are
certain beds which occur in the island of Seeland, in Denmark,
and which are known as the _Faxöe Limestone_.
Of a somewhat later date than the Maestricht beds is the _Pisolitic
Limestone_ of France, which rests unconformably on the White
Chalk, and contains a large number of Tertiary fossils along with
some characteristic Cretaceous types.
The subjoined sketch-section exhibits the general succession of
the Cretaceous deposits in Britain:--
[Illustration: Fig. 185. GENERALIZED SECTION OF THE CRETACEOUS
SERIES OF BRITAIN.]
In North America, strata of Lower Cretaceous age are well represented
in Missouri, Wyoming, Utah, and in some other areas; but the greater
portion of the American deposits of this period are referable to
the Upper Cretaceous. The rocks of this series are mostly sands,
clays, and limestones--_Chalk_ itself being unknown except in
Western Arkansas. Amongst the sandy accumulations, one of the
most important is the so-called "marl" of New Jersey, which is
truly a "Greensand," and contains a large proportion of glauconite
(silicate of iron and potash). It also contains a little phosphate
of lime, and is largely worked for agricultural purposes. The
greatest thickness attained by the Cretaceous rocks of North
America is about 9000 feet, as in Wyoming, Utah, and Colorado.
According to Dana, the Cretaceous rocks of the Rocky Mountain
territories pass upwards "without interruption into a coal-bearing
formation, several thousand feet thick, on which the following
Tertiary strata lie unconformably." The lower portion of this
"Lignitic formation" appears to be Cretaceous, and contains one
or more beds of Coal; but the upper part of it perhaps belongs
to the Lower Tertiary. In America, therefore, the lowest Tertiary
strata appear to rest conformably upon the highest Cretaceous;
whereas in Europe, the succession at this point is invariably an
unconformable one. Owing, however, to the fact that the American
"Lignitic formation" is a shallow-water formation, it can hardly
be expected to yield much material whereby to bridge over the
great palæontological gap between the White Chalk and Eocene
in the Old World.
Owing to the fact that so large a portion of the Cretaceous formation
has been deposited in the sea, much of it in deep water, the _plants_
of the period have for the most part been found special members
of the series, such as the Wealden beds, the Aix-la-Chapelle
sands, and the Lignitic beds of North America. Even the purely
marine strata, however, have yielded plant-remains, and some of
these are peculiar and proper to the deep-sea deposits of the
series. Thus the little calcareous discs termed "coccoliths," which
are known to be of the nature of calcareous sea-weeds (_Algoe_)
have been detected in the White Chalk; and the flints of the same
formation commonly contain the spore-cases of the microscopic
_Desmids_ (the so-called Xanthidia), along with the siliceous
cases of the equally diminutive _Diatoms_.
The plant-remains of the Lower Cretaceous greatly resemble those
of the Jurassic period, consisting mainly of Ferns, Cycads, and
Conifers. The Upper Cretaceous rocks, however, both in Europe and
in North America, have yielded an abundant flora which resembles
the existing vegetation of the globe in consisting mainly of
Angiospermous Exogens and of Monocotyledons.[23] In Europe the
plant-remains in question have been found chiefly in certain
sands in the neighbourhood of Aix-la-Chapelle, and they consist
of numerous Ferns, Conifers (such as _Cycadopteris_), Screw Pines
(_Pandanus_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_),
and many _Proteaceoe_, some of which are referred to existing
genera (_Dryandra, Banksia, Grevillea_, &c.)
[Footnote 23: The "Flowering plants" are divided into the two
great groups of the Endogens and Exogens. The _Endogens_ (such
as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of
growth, and the stem is said to be "endogenous;" the young plant
also possesses but a single seed-leaf or "cotyledon." Hence these
plants are often simply called "_Monocotyledons_." The _Exogens_,
on the other hand, have a true bark; and the stem increases by
annual additions to the outside, so that rings of growth are
produced. The young plant has two seed-leaves or "cotyledons,"
and these plants are therefore called "_Dicotyledons_." Amongst the
Exogens, the Pines (_Conifers_) and the Cycads have seeds which
are unprotected by a seed-vessel, and they are therefore called
"_Gymnosperms_." All the other Exogens, including the ordinary
trees, shrubs, and flowering plants, have the seeds enclosed in
a seed-vessel, and are therefore called "_Angiosperms_." The
derivation of these terms will be found in the Glossary at the
end of the volume.]
In North America, the Cretaceous strata of New Jersey, Alabama,
Nebraska, Kansas, &c., have yielded the remains of numerous plants,
many of which belong to existing genera. Amongst these may be
mentioned Tulip-trees (_Liriodendron_), Sassafras (fig. 186),
Oaks (_Quercus_), Beeches (_Fagus_), Plane-trees (_Platanus_),
Alders (_Alnus_), Dog-wood (_Cornus_), Willows (_Salix_), Poplars
(_Populus_), Cypresses (_Cupressus_), Bald Cypresses (_Taxodium_),
Magnolias, &c. Besides these, however, there occur other forms
which have now entirely disappeared from North America--as, for
example, species of _Cinnamomum_ and _Araucaria_.
It follows from the above, that the Lower and Upper Cretaceous
rocks are, from a botanical point of view, sharply separated
from one another. The Palæozoic period, as we have seen, is
characterised by the prevalance of "Flowerless" plants
(_Cryptogams_), its higher vegetation consisting almost exclusively
of Conifers. The Mesozoic period, as a whole, is characterised
by the prevalence of the Cryptogamic group of the Ferns, and
the Gymnospermic groups of the Conifers and the Cycads. Up to
the close of the Lower Cretaceous, no Angiospermous Exogens are
certainly known to have existed, and Monocotyledonous plants or
Endogens are very poorly represented. With the Upper Cretaceous,
however, a new era of plant-life, of which our present is but
the culmination, commenced, with a great and apparently sudden
development of new forms. In place of the Ferns, Cycads, and
Conifers of the earlier Mesozoic deposits, we have now an
astonishingly large number of true Angiospermous Exogens, many
of them belonging to existing types; and along with these are
various Monocotyledonous plants, including the first examples of
the great and important group of the Palms. It is thus a matter
of interest to reflect that plants closely related to those now
inhabiting the earth, were in existence at a time when the ocean
was tenanted by Ammonites and Belemnites, and when land and sea
and air were peopled by the extraordinary extinct Reptiles of
the Mesozoic period.
[Illustration: Fig. 186.--Cretaceous Angiosperms. a. _Sassafras
Cretaceum; b, Liriodendron Meekii; c, Leguminosites Marcouanus;
d, Salix Meekii_. (After Dana.)]
As regards animal life, the _Protozoans_ of the Cretaceous period
are exceedingly numerous, and are represented by _Foraminifera_
and _Sponges_. As we have already seen, the White Chalk itself is
a deep-sea deposit, almost entirely composed of the microscopic
shells of _Foraminifers_, along with Sponge-spicules, and organic
_débris_ of different kinds (see fig. 7). The green grains which
are so abundant in several minor subdivisions of the Cretaceous,
are also in many instances really casts in glauconite of the
chambered shells of these minute organisms. A great many species
of _Foraminifera_ have been recognised in the Chalk; but the
three principal genera are _Globigerina, Rotalia_ (fig. 187),
and _Textularia_--groups which are likewise characteristic of
the "ooze" of the Atlantic and Pacific Oceans at great depths.
The flints of the Chalk also commonly contain the shells of
_Foraminifera_. The Upper Greensand has yielded in considerable
numbers the huge _Foraminifera_ described by Dr Carpenter under
the name of _Parkeria_, the spherical shells of which are composed
of sand-grains agglutinated together, and sometimes attain a
diameter of two and a quarter inches. The Cretaceous Sponges are
extremely numerous, and occur under a great number of varieties
of shape and structure; but the two most characteristic genera
are _Siphonia_ and _Ventriculites_, both of which are exclusively
confined to strata of this age. The _Siphonioe_ (fig. 188) consist
of a pear-shaped, sometimes lobed head, supported by a longer
or shorter stern, which breaks up at its base into a number of
root-like processes of attachment. The water gained access to the
interior of the Sponge by a number of minute openings covering
the surface, and ultimately escaped by a single, large,
chimney-shaped aperture at the summit. In some respects these
sponges present a singular resemblance to the beautiful "Vitreous
Sponges" (_Holtenia_ or _Pheronema_) of the deep Atlantic; and,
like these, they were probably denizens of a deep sea, The
_Ventriculites_ of the Chalk (fig. 189) is, however, a genus
still more closely allied to the wonderful flinty Sponges, which
have been shown, by the researches of the Porcupine, Lightning,
and Challenger expeditions, to live half buried in the Calcareous
ooze of the abysses of our great oceans. Many forms of this genus
are known, having "usually the form of graceful vases, tubes, or
funnels, variously ridged or grooved, or otherwise ornamented
on the surface, frequently expanded above into a cup-like lip,
and continued below into a bundle of fibrous roots. The minute
structure of these bodies shows an extremely delicate tracery
of fine tubes, sometimes empty, sometimes filled with loose
calcareous matter dyed with peroxide of iron."--(Sir Wyville
Thomson.) Many of the Chalk sponges, originally calcareous, have
been converted into flint subsequently; but the Ventriculites
are really composed of this substance, and are therefore genuine
"Siliceous Sponges," like the existing Venus's Flower-Basket
(_Euplectella_). Like the latter, the skeleton was doubtless
originally composed, in the young state, of disconnected six-rayed
spicules, which ultimately become fixed together to constitute
a continuous frame-work. The sea-water, as in the recent forms,
must have been admitted to the interior of the Sponge by numerous
apertures on its exterior, subsequently escaping by a single
large opening at its summit.
[Illustration: Fig. 187--_Kotalia Boueana_.]
[Illustration: Fig. 188.--_Siphonia ficus. Upper Greensand. Europe.]
[Illustration: Fig. 189.--_Ventriculites simplex_. White Chalk.
Britain.]
Amongst the _Coelenterates_, the "Hydroid Zoophytes" are represented
by a species of the encrusting genus _Hydractinia_, the horny
polypary of which is so commonly found at the present day adhering
to the exterior of shells. The occurrence of this genus is of
interest, because it is the first known instance in the entire
geological series of the occurrence of an unquestionable Hydroid of
a modern type, though many of the existing forms of these animals
possess structures which are perfectly fitted for preservation in
the fossil condition. The corals of the Cretaceous series are not
very numerous, and for the most part are referable to types such
as _Trochocyathus, Stephanophyllia, Parasmilia, Synhelia_ (fig.
190), &c., which belong to the same great group of corals as the
majority of existing forms. We have also a few "Tabulate Corals"
(_Polytremacis_), hardly, if at all, generically separable from very
ancient forms (_Heliolites_); and the Lower Greensand has yielded
the remains of the little _Holocystis elegans_, long believed to
be the last of the great Palæozoic group of the _Rugosa_.
[Illustration: Fig. 190.--_Synhelia Sharpeana_. Chalk, England.]
As regards the _Echinoderms_, the group of the _Crinoids_ now
exhibits a marked decrease in the number and variety of its types.
The "stalked" forms are represented by _Pentacrinus_ and
_Bourgueticrinus_, and the free forms by Feather-stars like our
existing _Comatuloe_; whilst a link between the stalked and free
groups is constituted by the curious "Tortoise Encrinite
(_Marsupites_). By far the most abundant Cretaceous Echinoderms,
however, are Sea-urchins (_Echinoids_); though several Star-fishes
are known as well. The remains of Sea-urchins are so abundant
in various parts of the Cretaceous series, especially in the
White Chalk, and are often so beautifully preserved, that they
constitute one of the most marked features of the fauna of the
period. From the many genera of Sea-urchins which occur in strata
of this age, it is difficult to select characteristic types;
but the genera _Galerites_ (fig. 191), _Discoidea_ (fig. 192),
_Micraster, Ananchytes, Diadema, Salenia_, and _Cidaris_, may
be mentioned as being all important Cretaceous groups.
Coming to the _Annulose Animals_ of the Cretaceous period, there
is little special to remark. The _Crustaceans_ belong for the
most part to the highly-organised groups of the Lobsters and the
Crabs (the Macrurous and Brachyurous Decapods); but there are
also numerous little _Ostracodes_, especially in the fresh-water
strata of the Wealden. It should further be noted that there
occurs here a great development of the singular _Crustaceous_
family of the Barnacles (_Lepadidoe_), whilst the allied family
of the equally singular Acorn-shells (_Balanidoe_) is feebly
represented as well.
[Illustration: Fig. 191.--_Galerites albogalerus_, viewed from
below, from the side, and from above. White Chalk.]
[Illustration: Fig. 192.--_Discoidea cylindrica_; under, side,
and upper aspect. Upper Greensand.]
Passing on to the _Mollusca_, the class of the Sea-mats and
Sea-mosses (_Polyzoa_) is immensely developed in the Cretaceous
period, nearly two hundred species being known to occur in the
Chalk. Most of the Cretaceous forms belong to the family of the
_Escharidoe_, the genera _Eschara_ and _Escharina_ (fig. 193)
being particularly well represented. Most of the Cretaceous
_Polyzoans_ are of small size, but some attain considerable
dimensions, and many simulate Corals in their general form and
appearance.
The Lamp-shells (_Brachiopods_) have now reached a further stage
of the progressive decline, which they have been undergoing ever
since the close of the Palæozoic period. Though individually not
rare, especially in certain minor subdivisions of the series,
the number of generic types has now become distinctly diminished,
the principal forms belonging to the genera _Terebratula,
Terebratella_ (fig. 194), _Terebratulina, Rhynchonella_, and
_Crania_ (fig. 195). In the last mentioned of these, the shell
is attached to foreign bodies by the substance of one of the
valves (the ventral), whilst the other or free valve is more
or less limpet-shaped. All the above-mentioned genera are in
existence at the present day; and one _species_--namely,
_Terebratulina striata_--appears to be undistinguishable from
one now living--the _Terebratulina caputserpentis_.
[Illustration: Fig. 193.--A small fragment of _Escharina Oceani_,
of the natural size; and a portion of the same enlarged. Upper
Greensand.]
[Illustration: Fig. 194.--_Terebratella Astieriana_. Gault.]
Whilst the Lamp-shells are slowly declining, the Bivalves
(_Limellibranchs_) are greatly developed, and are amongst the
most abundant and characteristic fossils of the Cretaceous period.
In the great river-deposit of the Wealden, the Bivalves are forms
proper to fresh water, belonging to the existing River-mussels
(_Unio_), _Cyrena_ and _Cyclas_; but most of the Cretaceous
Lamellibranchs are marine. Some of the most abundant and
characteristic of these belong to the great family of the Oysters
(_Ostreidoe_). Amongst these are the genera _Gryphtoea_ and
_Exogyra_, both of which we have seen to occur abundantly in the
Jurassic; and there are also numerous true Oysters (_Ostrea_,
fig. 196) and Thorny Oysters (_Spondylus_, fig. 197). The genus
_Trigonia_, so characteristic of the Mesozoic deposits in general,
is likewise well represented in the Cretaceous strata. No single
genus of Bivalves is, however, so highly characteristic of the
Cretaceous period as _Inoceramus_, a group belonging to the family
of the Pearl-mussels (_Aviculidoe_). The shells of this genus
(fig. 198) have the valves unequal in size, the larger valve often
being much twisted, and both valves being marked with radiating
ribs or concentric furrows. The hinge-line is long and straight,
with numerous pits for the attachment of the ligament which serves
to open the shell. Some of the _Inocerami_ attain a length of
two or three feet, and fragments of the shell are often found
perforated by boring Sponges. Another extraordinary family of
Bivalves, which is exclusively confined to the Cretaceous rocks,
is that of the _Hippuritidoe_. All the members of this group
(fig. 199) were attached to foreign objects, and lived associated
in beds, like Oysters. The two valves of the shell are always
altogether unlike in sculpturing, appearance, shape, and size;
and the cast of the interior of the shell is often extremely
unlike the form of the outer surface. The type-genus of the family
is _Hippurites_ itself (fig. 199), in which the shell is in the
shape of a straight or slightly-twisted horn, sometimes a foot
or more in length, constituted by the attached lower valve, and
closed above by a small lid-like free upper valve. About a hundred
species of the family of the _Hippuritidoe_ are known, all of these
being Cretaceous, and occurring in Britain (one species only), in
Southern Europe, the West Indies, North America, Algeria, and
Egypt. Species of this family occur in such numbers in certain
compact marbles in the south of Europe, of the age of the Upper
Cretaceous (Lower Chalk), as to have given origin to the name
of "Hippurite Limestones," applied to these strata.
[Illustration: Fig. 195.--_Crania Ignabergensis_. The left-hand
figure shows the perfect shell, attached by its ventral valve
to a foreign body; the middle figure shows the exterior of the
limpet-shaped dorsal valve; and the right-hand figure represents
the interior of the attached valve. White Chalk.]
[Illustration: Fig. 196.--_Ostrea Couloni_. Lower Greensand.]
[Illustration: Fig. 197.--_Spondylus spinosus_. White Chalk.]
[Illustration: Fig. 198.--_Inoceramus sulcatus_. Gault.]
The Univalves (_Gasteropods_) of the Cretaceous period are not
very numerous, nor particularly remarkable. Along with species of
the persistent genus _Pleurotomaria_ and the Mesozoic _Nerinoea_,
we meet with examples of such modern types as _Turritella_ and
_Natica_, the Staircase-shells (_Solarium_), the Wentle-traps
(_Scalaria_), the Carrier-shells (_Phorus_), &c. Towards the close
of the Cretaceous period, and especially in such transitional strata
as the Maestricht beds, the Faxöe Limestone, and the Pisolitic
Limestone of France, we meet with a number of carnivorous
("siphonostomatous") Univalves, in which the mouth of the shell is
notched or produced into a canal. Amongst these it is interesting
to recognise examples of such existing genera as the Volutes
(_Voluta_, fig. 200), the Cowries (_Cyproea_), the Mitre-shells
(_Mitra_), the Wing - shells (_Strombus_), the Scorpion-shells
(_Pteroceras_), &c.
[Illustration: Fig. 199.--_Hippurites Toucasiana_. A large
individual, with two smaller ones attached to it. Upper Cretaceous,
South of Europe.]
[Illustration: Fig. 200.--_Voluta elongata_. White Chalk.]
Upon the whole, the most characteristic of all the Cretaceous
Molluscs are the _Cephalopods_, represented by the remains of
both _Tetrabranchiate_ and _Dibranchiate_ forms. Amongst the
former, the long-lived genus _Nautilus_ (fig. 201) again reappears,
with its involute shell, its capacious body-chamber, its simple
septa between the air-chambers, and its nearly or quite central
siphuncle. The majority of the chambered _Cephalopods_ of the
Cretaceous belong, however, to the complex and beautiful family
of the _Ammonitidoe_, with their elaborately folded and lobed
septa and dorsally-placed siphuncle. This family disappears wholly
at the close of the Cretaceous period; but its approaching
extinction, so far from being signalised by any slow decrease
and diminution in the number of specific or generic types, seems
to have been attended by the development of whole series of new
forms. The genus _Ammonites_ itself, dating from the Carboniferous,
has certainly passed its prime, but it is still represented by
many species, and some of these attained enormous dimensions
(two or three feet in diameter). The genus _Ancyloceras_ (fig.
202), though likewise of more ancient origin (Jurassic), is
nevertheless very characteristic of the Cretaceous. In this genus
the first portion of the shell is in the form of a flat spiral,
the coils of which are not in contact; and its last portion is
produced at a tangent, becoming ultimately bent back in the form
of a crosier. Besides these pre-existent types, the Cretaceous
rocks have yielded a great number of entirely new forms of the
_Ammonitidoe_, which are not known in any deposits of earlier or
later date. Amongst the more important of these may be mentioned
_Crioceras, Turrilites, Scaphites, Hamites, Ptychoceras_, and
_Baulites_. In the genus _Crioceras_ (fig. 204, d), the shell
consists of an open spiral, the volutions of which are not in
contact, thus resembling a partially-unrolled _Ammonite_ or the
inner portion of an _Ancyloceras_. In _Turrilites_ (fig. 203), the
shell is precisely like that of the _Ammonite_ in its structure;
but instead of forming a flat spiral, it is coiled into an elevated
turreted shell, the whorls of which are in contact with one another.
In the genus _Scaphites_ (fig. 204, e), the shell resembles that
of _Ancyloceras_ in consisting of a series of volutions coiled
into a flat spiral, the last being detached from the others,
produced, and ultimately bent back in the form of a crosier; but
the whorls of the enrolled part of the shell are in contact,
instead of being separate as in the latter. In the genus _Hamites_
(fig. 204, f), the shell is an extremely elongated cone, which
is bent upon itself more than once, in a hook-like manner, all
the volutions being separate. The genus _Ptychoteras_ (fig. 204,
a) is very like _Hamites_, except that the shell is only bent
once; and the two portions thus bent are in contact with one
another. Lastly, in the genus _Baculites_ (fig. 204, b and
c) the shell is simply a straight elongated cone, not bent
in any way, but possessing the folded septa which characterise
the whole Ammonite family. The _Baculite_ is the simplest of
all the forms of the _Ammonitidoe_; and all the other forms,
however complex, may be regarded as being simply produced by the
bending or folding of such a conical septate shell in different
ways. The _Baculite_, therefore, corresponds, in the series of
the _Ammonitidoe_, to the _Orthoceras_ in the series of the
_Nautilidoe_. All the above-mentioned genera are characteristically,
or exclusively, Cretaceous, and they are accompanied by a number
of other allied forms, which cannot be noticed here. Not a single
one of these genera, further, has hitherto been detected in any
strata higher than the Cretaceous. We may therefore consider that
these wonderful, varied, and elaborate forms of _Ammonitidoe_
constitute one of the most conspicuous features in the life of
the Chalk period.
[Illustration: Fig. 201.--Different views of _Nautilus Danicus_.
Faxöe Limestone (Upper Cretaceous), Denmark.]
[Illustration: Fig. 202.--_Ancyloceras Matheronianus_. Gault.]
The _Dibranchiate Cephalopods_ are represented partly by the
beak-like jaws of unknown species of Cuttle-fishes and partly
by the internal skeletons of Belemnites. Amongst the latter,
the genus _Belemnites_ itself holds its place in the lower part
of the Cretaceous series; but it disappears in the upper portion
of the series, and its place is taken by the nearly-allied genus
_Belemnitella_ (fig. 205), distinguished by the possession of
a straight fissure in the upper end of the guard. This also
disappears at the close of the Cretaceous period; and no member
of the great Mesozoic family of the _Belemnitidoe_ has hitherto
been discovered in any Tertiary deposit, or is known to exist
at the present day.
[Illustration: Fig. 203.--_Turrilites catenatus_. The lower figure
represents the entire shell; the upper figure represents the
base of the shell seen from below. Gault.]
[Illustration: Fig. 204.--a, _Ptychoceras Emericianum_,
reduced--Lower Greensand; b, _Baculites anceps_, reduced--Chalk;
c, Portion of the same, showing the folded edges of the septa;
d, _Crioceras cristatum_, reduced--Gault; e, _Scaphites oequalis_,
natural size--Chalk; f, _Hamites rotundus_, restored--Gault.]
Passing on next to the _Vertebrate Animals_ of the Cretaceous
period, we find the _Fishes_ represented as before by the Ganoids
and the Placoids, to which, however, we can now add the first
known examples of the great group of the _Bony Fishes_ or
_Teleosteans_, comprising the great majority of existing forms.
The _Ganoid_ fishes of the Cretaceous (_Lepidotus, Pycnodus_,
&c.) present no features of special interest. Little, also, need
be said about the _Placoid_ fishes of this period. As in the
Jurassic deposits, the remains of these consist partly of the
teeth of genuine Sharks (_Lamna, Odontaspis_, &c.) and partly
of the teeth and defensive spines of Cestracionts, such as the
living Port-Jackson Shark. The pointed and sharp-edged teeth of
true Sharks are very abundant in some beds, such as the Upper
Greensand, and are beautifully preserved. The teeth of some forms
(_Carcharias_, &c.) attain occasionally a length of three or four
inches, and indicate the existence in the Cretaceous seas of
huge predaceous fishes, probably larger than any existing Sharks.
The remains of _Cestracionts_ consist partly of the flattened
teeth of genera such as _Acrodus_ and _Ptychodus_ (the latter
confined to rocks of this age), and partly of the pointed teeth
of _Hybodus_, a genus which dates from the Trias. In this genus
the teeth (fig. 206) consist of a principal central cone, flanked
by minor lateral cones; and the fin-spines (fig. 207) are
longitudinally grooved, and carry a series of small spines on
their hinder or concave margin. Lastly, the great modern order
of the Bony Fishes or _Teleosteans_ makes its first appearance
in the Upper Cretaceous rocks, where it is represented by forms
belonging to no less than three existing groups--namely, the
Salmon family (_Salmonidoe_), the Herring family (_Clupeidoe_),
and the Perch family (_Percidoe_). All these fishes have thin,
horny, overlapping scales, symmetrical ("homocercal") tails,
and bony skeletons. The genus _Beryx_ (fig. 208, 1) is one
represented by existing species at the present day, and belongs
to the Perch family. The genus _Osmeroides_, again (fig. 208,
2), is supposed to be related to the living Smelts (_Osmerus_),
and, therefore, to belong to the Salmon tribe.
[Illustration: Fig. 205.--Guard of _Belemnitella mucronata_. White
Chalk.]
[Illustration: Fig. 206.--Tooth of _Hybodus_.]
[Illustration: Fig. 207.--Fin-spine of _Hybodus_. Lower Greensand.]
[Illustration: Fig. 208.--1, _Beryx Lewesiensis_, a Percoid fish
from the Chalk; 2, _Osmeroides Mantelli_, a Salmonoid fish from
the Chalk.]
No remains of _Amphibians_ have hitherto been detected in any part
of the Cretaceous series; but _Reptiles_ are extremely numerous,
and belong to very varied types. As regards the great extinct groups
of Reptiles which characterise the Mesozoic period as a whole, the
huge "Enaliosaurs" or "Sea-Lizards" are still represented by the
_Ichthyosaur_ and the _Plesiosaur_. Nearly allied to the latter
of these is the _Elasmosaurus_ of the American Cretaceous, which
combined the long tail of the Ichthyosaur with the long neck
of the Plesiosaur. The length of this monstrous Reptile could not
have been less than fifty feet, the neck consisting of over sixty
vertebræ and measuring over twenty feet in length. The extraordinary
Flying Reptiles of the Jurassic are likewise well represented in
the Cretaceous rocks by species of the genus _Pterodactylus_
itself, and these later forms are much more gigantic in their
dimensions than their predecessors. Thus some of the Cretaceous
Pterosaurs seem to have had a spread of wing of from twenty to
twenty-five feet, more than realising the "Dragons" of fable in
point of size. The most remarkable, however, of the Cretaceous
_Pterosaurs_ are the forms which have recently been described
by Professor Marsh under the generic title of _Pteranodon_. In
these singular forms--so far only known as American--the animal
possessed a skeleton in all respects similar to that of the typical
Pterodactyles, except that the jaws are completely destitute of
teeth. There is, therefore, the strongest probability that the
jaws were encased in a horny sheath, thus coming to resemble the
beak of a Bird. Some of the recognised species of _Pteranodon_
are very small; but the skull of one species (_P. Longiceps_)
is not less than a yard in length, and there are portions of
the skull of another species which would indicate a length of
four feet for the cranium. These measurements would point to
dimensions larger than those of any other known Pterosaurs.
The great Mesozoic order of the _Deinosaurs_ is largely represented
in the Cretaceous rocks, partly by genera which previously existed
in the Jurassic period, and partly by entirely new types. The great
delta-deposit of the Wealden, in the Old World, has yielded the
remains of various of these huge terrestrial Reptiles, and very
many others have been found in the Cretaceous deposits of North
America. One of the most celebrated of the Cretaceous Deinosaurs
is the _Iguanodon_, so called from the curious resemblance of
its teeth to those of the existing but comparatively diminutive
_Iguana_. The teeth (fig. 209) are soldered to the inner face
of the jaw, instead of being sunk in distinct sockets; and they
have the form of somewhat flattened prisms, longitudinally ridged
on the outer surface, with an obtusely triangular crown, and
having the enamel crenated on one or both sides. They present
the extraordinary feature that the crowns became worn down flat
by mastication, showing that the _Iguanodon_ employed its teeth
in actually chewing and triturating the vegetable matter on which
it fed. There can therefore be no doubt but that the _Iguanodon_,
in spite of its immense bulk, was an herbivorous Reptile, and lived
principally on the foliage of the Cretaceous forests amongst which
it dwelt. Its size has been variously estimated at from thirty to
fifty feet, the thigh-bone in large examples measuring nearly
five feet in length, with a circumference of twenty-two inches
in its smallest part. With the strong and massive hind-limbs are
associated comparatively weak and small fore-limbs; and there
seems little reason to doubt that the _Iguanodon_ must have walked
temporarily or permanently upon its hind-limbs, after the manner of
a Bird. This conjecture is further supported by the occurrence in
the strata which contain the bones of the _Iguanodon_ of gigantic
three-toed foot-prints, disposed _singly_ in a double track. These
prints have undoubtedly been produced by some animal walking on
two legs; and they can hardly, with any probability, be ascribed
to any other than this enormous Reptile. Closely allied to the
_Iguanodon_ is the _Hadrosaurus_ of the American Cretaceous, the
length of which is estimated at twenty-eight feet. _Iguanodon_
does not appear to have possessed any integumentary skeleton; but
the great _Hyloeosaurus_ of the Wealden seems to have been furnished
with a longitudinal crest of large spines running down the back,
similar to that which is found in the comparatively small Iguanas
of the present day. The _Megalosaurus_ of the Oolites continued
to exist in the Cretaceous period; and, as we have previously
seen, it was carnivorous in its habits. The American _Loelaps_
was also carnivorous, and, like the Megalosaur, which it very
closely resembles, appears to have walked upon its hind-legs,
the fore-limbs being disproportionately small.
[Illustration: Fig. 209.--Teeth of Iguanodon Mantellii. Wealden,
Britain.]
Another remarkable group of Reptiles, exclusively confined to
the Cretaceous series, is that of the _Mosasauroids_, so called
from the type-genus _Mosasaurus_. The first species of _Mosasaurus_
known to science was the _M. Camperi_ (fig. 210), the skull of
which--six feet in length--was discovered in 1780 in the Maestricht
Chalk at Maestricht. As this town stands on the river Meuse,
the name of _Mosasaurus_ ("Lizard of the Meuse") was applied
to this immense Reptile. Of late years the remains of a large
number of Reptiles more or less closely related to _Mosasaurus_, or
absolutely belonging to it, have been discovered in the Cretaceous
deposits of North America, and have been described by Professors
Cope and Marsh. All the known forms of this group appear to have
been of large size--one of them, _Mosasaurus princeps_, attaining
the length of seventy-five or eighty feet, and thus rivalling
the largest of existing Whales in its dimensions. The teeth in
the "Mosasauroids" are long, pointed, and slightly curved; and
instead of being sunk in distinct sockets, they are firmly
amalgamated with the jaws, as in modern Lizards. The palate also
carried teeth, and the lower jaw was so constructed as to allow
of the mouth being opened to an immense width, somewhat as in the
living Serpents. The body was long and snake-like, with a very
long tail, which is laterally compressed, and must have served as
a powerful swimming-apparatus. In addition to this, both pairs
of limbs have the bones connecting them with the trunk greatly
shortened; whilst the digits were enclosed in the integuments,
and constituted paddles, closely resembling in structure the
"flippers" of Whales and Dolphins. The neck is sometimes moderately
long, but oftener very short, as the great size and weight of
the head would have led one to anticipate. Bony plates seem in
some species to have formed an at any rate partial covering to
the skin; but it is not certain that these integumentary appendages
were present in all. Upon the whole, there can be no doubt but
that the Mosasauroid Reptiles--the true "Sea-serpents" of the
Cretaceous period--were essentially aquatic in their habits,
frequenting the sea, and only occasionally coming to the land.
[Illustration: Fig. 210.--Skull of _Mosasaurus Camperi_, greatly
reduced. Maestricht Chalk.]
The "Mosasauroids" have generally been regarded as a greatly
modified group of the Lizards (_Lacertilia_). Whether this reference
be correct or not--and recent investigations render it dubious--the
Cretaceous rocks have yielded the remains of small Lizards not widely
removed from existing forms. The recent order of the _Chelonians_
is also represented in the Cretaceous rocks, by forms closely
resembling living types. Thus the fresh-water deposits of the
Wealden have yielded examples of the "Terrapins" or "Mud-Turtles"
(_Emys_); and the marine Cretaceous strata have been found to
contain the remains of various species of Turtles, one of which
is here figured (fig. 211). No true Serpents (_Ophidia_) have
as yet been detected in the Cretaceous rocks; and this order
does not appear to have come into existence till the Tertiary
period. Lastly, true Crocodiles are known to have existed in
considerable numbers in the Cretaceous period. The oldest of
these occur in the fresh-water deposit of the Wealden; and they
differ from the existing forms of the group in the fact that the
bodies of the vertebræ, like those of the Jurassic Crocodiles,
are bi-concave, or hollowed out at both ends. In the Greensand
of North America, however, occur the remains of Crocodiles which
agree with all the living species in having the bodies of the
vertebræ in the region of the back hollowed out in front and
convex behind.
[Illustration: Fig. 211.--Carapace of _Chelone Benstedi_. Lower
Chalk. (After Owen.)]
_Birds_ have not hitherto been shown, with certainty, to have
existed in Europe during the Cretaceous period, except in a few
instances in which fragmentary remains belonging to this class
have been discovered. The Cretaceous deposits of North America
have, however, been shown by Professor Marsh to contain a
considerable number of the remains of Birds, often in a state
of excellent preservation. Some of these belong to Swimming or
Wading Birds, differing in no point of special interest from
modern birds of similar habits. Others, however, exhibit such
extraordinary peculiarities that they merit more than a passing
notice. One of the forms in question constitutes the genus
_Ichthyornis_ of Marsh, the type-species of which (_I. Dispar_)
was about as large as a Pigeon. In two remarkable respects, this
singular Bird differs from all known living members of the class.
One of these respects concerns the jaws, both of which exhibit the
Reptilian character of being armed with numerous small pointed
_teeth_ (fig. 212, a), sunk in distinct sockets. No existing
bird possesses teeth; and this character forcibly recalls the
Bird-like Pterosaurs, with their toothed jaws. _Ichthyornis_,
however, possessed fore-limbs constructed strictly on the type
of the "wing" of the living Birds; and it cannot, therefore, be
separated from this class. Another extraordinary peculiarity
of _Ichthyornis_ is, that the bodies of the _vertebrie_ (fig.
212, c) were _bi-concave_, as is the case with many extinct
Reptiles and almost all Fishes, but as does not occur in any
living Bird. There can be little doubt that _Ichthyornis_ was
aquatic in its habits, and that it lived principally upon fishes;
but its powerful wings at the same time indicate that it was
capable of prolonged flight. The tail of _Ichthyornis_ has,
unfortunately, not been discovered; and it is at present impossible
to say whether this resembled the tail of existing Birds, or
whether it was elongated and composed of separate vertebræ, as
in the Jurassic _Archoeopteryx_.
Still more wonderful than _Ichthyornis_ is the marvellous bird
described by Marsh under the name of _Hesperornis regalis_. This
presents us with a gigantic diving bird, somewhat resembling the
existing "Loons" (_Colymbus_), but agreeing with _Ichthyornis_
in having the jaws furnished with conical, recurved, pointed
teeth (fig. 212, b). Hence these forms are grouped together in
a new sub-class, under the name of _Odontornithes_ or "Toothed
Birds." The teeth of _Hesperornis_ (fig. 212, d) resemble those
of _Ichthyornis_ in their general form; but instead of being
sunk in distinct sockets, they are simply implanted in a deep
continuous groove in the bony substance of the jaw. The front of
the upper jaw does not carry teeth, and was probably encased in
a horny beak. The breast-bone is entirely destitute of a central
ridge or keel, and the wings are minute and quite rudimentary; so
that _Hesperornis_, unlike _Ichthyornis_, must have been wholly
deprived of the power of flight, in this respect approaching the
existing Penguins. The tail consists of about twelve vertebræ,
of which the last three or four are amalgamated to form a flat
terminal mass, there being at the same time clear indications
that the tail was capable of up and down movement in a vertical
plane, this probably fitting it to serve as a swimming-paddle or
rudder. The legs were powerfully constructed, and the feet were
adapted to assist the bird in rapid motion through the water. The
known remains of _Hesperornis regalis_ prove it to have been a
swimming and diving bird, of larger dimensions than any of the
aquatic members of the class of Birds with which we are acquainted
at the present day. It appears to have stood between five and six
feet high, and its inability to fly is fully compensated for by the
numerous adaptations of its structure to a watery life. Its teeth
prove it to have been carnivorous in its habits, and it probably
lived upon fishes. It is a curious fact that two Birds agreeing
with one another in the wholly abnormal character of possessing
teeth, and in other respects so entirely different, should, like
_Ichthyornis_ and _Hesperornis_, have lived not only in the same
geological period, but also in the same geographical area; and
it is equally curious that the area inhabited by these toothed
Birds should at the same time have been tenanted by winged and
bird-like Reptiles belonging to the toothed genus _Pterodactylus_
and the toothless genus _Pteranodon_.
[Illustration: Fig. 212.--Toothed Birds (_Odontornithes_) of the
Cretaceous Rocks of America. a. Left lower jaw of _Ichthyornis
dispar_, slightly enlarged; b, Left lower jaw of _Hesperornis
regalis_, reduced to nearly one-fourth of the natural size; c.
Cervical vertebra of _Ichthyornis dispar_, front view, twice
the natural size; c', Side view of the same; d, Tooth of
_Hesperornis regalis_, enlarged to twice the natural size. (After
Marsh.)]
No remains of _Mammals_, finally, have as yet been detected in
any sedimentary accumulations of Cretaceous age.
LITERATURE.
The following list comprises some of the more important works and
memoirs which may be consulted with reference to the Cretaceous
strata and their fossil contents:--
(1) 'Memoirs of the Geological Survey of Great Britain.'
(2) 'Geology of England and Wales.' Conybeare and Phillips.
(3) 'Geology of Yorkshire,' vol. ii. Phillips.
(4) 'Geology of Oxford and the Thames Valley.' Phillips.
(5) 'Geological Excursions through the Isle of Wight.' Mantell.
(6) 'Geology of Sussex.' Mantell.
(7) 'Report on Londonderry,' &c. Portlock.
(8) 'Recherches sur le Terrain Crétacé Supérieur de l'Angleterre
et de l'Irlande.' Barrois.
(9) "Geological Survey of Canada"--'Report of Progress, 1872-73.'
(10) 'Geological Survey of California.' Whitney.
(11) 'Geological Survey of Montana, Idaho, Wyoming, and Utah.'
Hayden and Meek.
(12) 'Report on Geology,' &c. (British North American Boundary
Commission). G. M. Dawson.
(13) 'Manual of Geology.' Dana.
(14) 'Lethæa Rossica.' Eichwald.
(15) 'Petrefacta Germaniæ.' Goldfuss.
(16) 'Fossils of the South Downs.' Mantell.
(17) 'Medals of Creation.' Mantell.
(18) 'Mineral Conchology.' Sowerby.
(19) 'Lethæa Geognostica.' Bronn.
(20) 'Malacostracous Crustacea of the British Cretaceous Formation'
(Palæontographical Society). Bell.
(21) 'Brachiopoda of the Cretaceous Formation' (Palæontographical
Society). Davidson.
(22) 'Corals of the Cretaceous Formation' (Palæontographical
Society). Milne-Edwards and Haime.
(23) 'Supplement to the Fossil Corals' (Palæontographical Society).
Martin Duncan.
(24) 'Echinodermata or the Cretaceous Formation' (Palæontographical
Society). Wright.
(25) 'Monograph of the Belemnitidæ' (Palæontographical Society).
Phillips.
(26) 'Monograph of the Trigoniæ' (Palæontographical Society).
Lycett.
(27) 'Fossil Cirripedes' (Palæontographical Society). Darwin.
(28) 'Fossil Mollusca of the Chalk of Britain' (Palæontographical
Society). Sharpe.
(29) 'Entomostraca of the Cretaceous Formation' (Palæontographical
Society). Rupert Jones.
(30) 'Monograph of the Fossil Reptiles of the Cretaceous Formation'
(Palæontographical Society). Owen.
(31) 'Manual of Palæontology.' Owen.
(32) 'Synopsis of Extinct Batrachia and Reptilia.' Cope.
(33) "Structure of the Skull and Limbs in Mosasauroid
Reptiles"--'American Journ. Sci. and Arts, 1872.' Marsh.
(34) "On Odontornithes"--'American Journ. Sci. and Arts, 1875.'
Marsh.
(35) 'Ossemens Fossiles.' Cuvier.
(36) 'Catalogue of Ornithosauria.' Seeley.
(37) 'Paléontologie Française.' D'Orbigny.
(38) 'Synopsis des Echinides fossiles.' Desor.
(39) 'Cat. Raisonné des Echinides.' Agassiz and Desor.
(40) "Echinoids"--'Decades of the Geol. Survey of Britain.'
E. Forbes.
(41) 'Paléontologie Française.' Cotteau.
(42) 'Versteinerungen der Böhmischen Kreide-formation.' Reuss.
(43) "Cephalopoda, Gasteropoda, Pelecypoda, Brachiopoda; &c., of the
Cretaceous Rocks of India"--'Palæontologica Indica,' ser. i.,
iii., v., vi., viii. Stoliczka.
(44) "Cretaceous Reptiles of the United States"--'Smithsonian
Contributions to Knowledge,' vol. xiv. Leidy.
(45) 'Invertebrate Cretaceous, and Tertiary Fossils of the Upper
Missouri Country,' 1876. Meek.