TRILOBITA
Among the many interesting groups of fossils found in the Palaeozoic
deposits there is none which has attracted more attention than the
Trilobites. As early as 1698, Edward Lhwyd, Curator of the Ashmolean
Museum in Oxford, recorded in the _Philosophical Transactions_ the
discovery of Trilobites in the neighbourhood of Llandeilo in South
Wales; and of one of his specimens he remarked that “it must be the
Sceleton of a flat Fish.” In the following year the same writer gave in
his _Lithophylacii Britannici Ichnographia_ descriptions and figures of
two Trilobites which are evidently examples of the species now known as
_Ogygia buchi_ and _Trinucleus fimbriatus_.
Although Trilobites differ so much from living Arthropods that it was
difficult to determine even whether they belonged to the Crustacea or
the Arachnida, yet one of the earliest writers, Dr. Cromwell Mortimer,
Secretary of the Royal Society (1753), recognised their resemblance to
_Apus_ (see pp. 19–36). This view of their affinities was adopted by
Linnaeus, and has been supported by many later writers. Another early
author, Emanuel Mendez da Costa, thought that the Trilobites were
related to the Isopods, an opinion which has been held by some few
zoologists of more recent times.
The Trilobites form the only known Order of the Crustacea which has no
living representatives. They are found in the oldest known fossiliferous
deposits—the Lower Cambrian or _Olenellus_ beds, where they are
represented by 19 genera belonging to the families Agnostidae,
Paradoxidae, Olenidae, and Conocephalidae. From the variety of forms
found and the state of development which they have reached, it is
evident that even at that remote period the group must have been of
considerable antiquity; but of its pre-Cambrian ancestors nothing is yet
known; consequently there is no direct evidence of the origin of the
group.
Trilobites form an important part of all the faunas of the Cambrian
system; they attain their greatest development in the Ordovician period,
after which they become less numerous; their decline is very marked in
the Devonian, in which nearly all the genera are but survivals from the
Silurian period; in the Carboniferous, evidence of approaching
extinction is seen in the small number of genera represented, all of
which belong to one family—the Proëtidae, in the relatively few species
in each genus and in the small size of the individuals of those species.
In Europe no representatives of the group appear to have survived the
Carboniferous period, but in America one form has been recorded from
deposits of Permian age.
Trilobites seem to have been exclusively marine, since they are found
only in association with the remains of marine animals. Their range in
depth was evidently considerable, for they occur in many different kinds
of sediment, and were apparently able to live regardless of the nature
of the sea-floor—whether muddy, sandy, calcareous, or rocky. In some
cases they occur in deposits containing reef-building corals and other
shallow water animals; in others they are associated with organisms
which lived at greater depths. The group appears to have had a
world-wide distribution, for the remains of Trilobites are found in the
Palaeozoic rocks of all countries. Their range in size is considerable;
for whilst a large proportion of the species are about two or three
inches in length, some, like _Agnostus_, are only a quarter of an inch
long, others are from ten to twenty inches long, the largest forms
including species of _Paradoxides_, _Asaphus_, _Megalaspis_, _Lichas_,
and _Homalonotus_.
The feature in a Trilobite which first attracts attention is the marked
division of the dorso-ventrally flattened body into a median or axial
part, and a lateral or pleural part on each side. It was this character
that led Walch, in 1771, to give the name by which the group is now
known. The axial part of the body contained the alimentary canal, as is
shown by the position of the mouth and anus, as well as by casts in mud
of the canal which are found in some specimens. The trilobation of the
body is quite distinct in the majority of Trilobites, but in a few
genera belonging to the Asaphidae and Calymenidae (Fig. 136) it becomes
more or less completely obsolete.
[Illustration:
FIG. 136.—_Homalonotus delphinocephalus_, Green, × 1. Silurian. (After
Zittel.)
]
In most cases the only part of the Trilobite which is preserved is the
exoskeleton which covered the dorsal surface of the body. That skeleton
consists largely of calcareous material, and shows in sections a finely
perforated structure. Generally it is arched above, but in some cases is
only slightly convex; in outline it is more or less oval. Three regions
can always be distinguished in the body of a Trilobite—the head, the
thorax, and the abdomen or pygidium.
The carapace which covers the =head= is known as the cephalic shield
(Fig. 137, A, 1), and is commonly more or less semicircular in outline,
but varies considerably in different genera. Only in a few cases, as in
some species of _Agnostus_ (Fig. 146), is its length greater than its
breadth. The axial part of the cephalic shield, called the “glabella”
(Fig. 137, A, _a_), is usually more convex than the lateral parts
(“cheeks” or “genae”), and is separated from them by longitudinal or
axial furrows (_b_). The shape of the glabella varies greatly; it may be
oblong, circular, semi-cylindrical, pyriform, spherical, etc. Its
relative size likewise varies; thus in _Phacops cephalotes_ it expands
in front and forms the larger part of the head, whilst in _Arethusina_
(Fig. 151, B) it is narrow and short, being only about one-half of the
length of the head.
[Illustration:
FIG. 137.—_Calymene tuberculata_, Brünn. × 1. Silurian, Dudley. =A=,
Dorsal surface: 1, head; 2, thorax; 3, pygidium or abdomen. _a_,
Glabella; _b_, axial furrow; _c_, glabella-furrow; _d_, neck-furrow;
_e_, fixed cheek; _f_, free cheek; _g_, facial suture; _h_, eye;
_i_, genal angle; _k_, axis of thorax; _l_, pleura. =B=, Ventral
surface of head (after Barrande): _a_, hypostome; _b_, doublure;
_c_, _c′_, facial sutures; _d_, rostral suture; _e_, rostral plate.
=C=, One segment of the thorax: _a_, ring of axis; _b_, groove; _c_,
articular portion; _d_, axial furrow; _d-f_, pleura; _d-e_, internal
part of pleura; _e-f_, external part of pleura; _e_, fulcrum; _g_,
groove. =D=, Coiled specimen: _a_, glabella; _b_, eye; _c_, facial
suture; _d_, pygidium; _e_, rostral suture; _f_, continuation of
facial suture.
]
The segmentation of the head is indicated by transverse furrows on the
glabella (Fig. 137, A, _c_, _d_). In some cases these furrows extend
quite across the glabella (Fig. 147), but commonly they are found on the
sides only and divide the glabella into lateral lobes. Only the
posterior or “neck-furrow” (Fig. 137, A, _d_) is continued on to the
cheeks, and the segment which it limits anteriorly on the glabella[180]
is known as the occipital or neck-ring. In front of the neck-furrow
there may be three other furrows, so that altogether five cephalic
segments are indicated by the furrows of the glabella. Commonly all the
furrows are distinct in the primitive types; but in the more modified
forms some, especially the anterior, become either reduced in size or
obsolete. The actual number of furrows present consequently varies in
different genera, and may even differ in different species of the same
genus. In a few genera all the furrows are either indistinct or absent,
as for example in _Ellipsocephalus_ (Fig. 150, B). In some cases four
furrows are present in addition to the neck-furrow; this is due to the
division of the anterior lobe of the glabella by fulcra which are
developed for the attachment of muscles.
When the glabella reaches the front border of the head the two cheeks
are separated (Fig. 150, I); but in other cases they unite in front of
the glabella (Fig. 150, C). The outer posterior angle of the cheeks or
genae (“genal angle,” Fig. 137, A, _i_) may be rounded, pointed, or
produced into backwardly directed spines (Fig. 140). The marginal part
of the cephalic shield is often flattened or concave; this border may be
quite a narrow rim as in _Calymene_ (Fig. 137, A), but in some genera
(e.g. _Trinucleus_, Fig. 140, B; _Harpes_, Fig. 150, A; _Asaphus_) it
attains a great development. Each cheek is usually divided by a suture—
the “facial suture” (Fig. 137, A, _g_)—into an inner and an outer part;
the former is the “fixed cheek” (_e_), and the latter the “free cheek”
(_f_). The course of the facial suture varies in different genera: on
the posterior part of the head it begins either at the posterior margin
(Fig. 150, C) or at the posterior part of the lateral margin (Fig. 151,
C, D); at first it is directed inwards, and then bends forward, forming
an angle. In front it may (_a_) end at the front margin (Fig. 147), or
(_b_) be united beneath the front margin by a rostral suture (Fig. 137,
B, _d_, D, _e_), or (_c_) unite with the other suture on the dorsal
surface in front of the glabella (Fig. 151, C). In the last case the
free cheeks also unite in front of the glabella.
The facial suture is one of the distinguishing features of the
Trilobites, and may have been of some use in ecdysis. In only a few
forms is it absent, as for example in _Agnostus_ (Fig. 146) and
_Microdiscus_. In the former, however, Beecher states that a suture is
really present, but, unlike that of most other Trilobites, it is
situated at the margin of the cephalic shield, and consequently the free
cheek, if present, must be on the ventral surface. Lindström and Holm,
after a re-examination of well-preserved specimens, deny the existence
of a suture in _Agnostus_. By most authors _Olenellus_ is said to be
without a suture, but Beecher maintains that although the fixed and free
cheeks have coalesced, yet a raised line passing from the eye-lobe to
the posterior margin marks the position of the suture; this view is not
accepted by Lindström.
The existence of a facial suture in _Trinucleus_ has likewise been
disputed. But Emmerich, Salter, and M‘Coy[181] have maintained that a
suture is present in a normal position on the dorsal surface, extending
from the posterior margin just within the genal angle to the eye (when
present), and from thence bending forward and ending on the front margin
near the glabella. It must be admitted that no indications of the suture
are seen in the majority of specimens, perhaps owing to the fact that
most examples of _Trinucleus_ are in the form of internal casts; perhaps
also to the more or less complete coalescence of the fixed and free
cheeks, since in no specimen has the free cheek been found separated
from the rest of the head, as occurs not uncommonly in many other
Trilobites. The probability of the existence of a suture receives some
support from the fact that one is found in the allied genera
_Orometopus_ and _Ampyx_ (Fig. 140). Barrande and Oehlert deny its
existence in _Trinucleus_. There is, however, in that genus a suture
running close to the margin of the cephalic border,[182] and joining the
genal angle so as to cut off the genal spine. Lovén and Oehlert claim
that this suture represents the facial suture, but in an abnormal
position; this view, however, is not accepted by Beyrich. In this
connection it should be noted that in _Acidaspis_, whilst the majority
of the species possess a facial suture, there are two in which it has
disappeared owing to the fusion of the fixed and free cheeks. Such being
the case, it seems not improbable that the curved line passing backwards
from the eye in _Harpes_ may mark the position of the suture; but it is
stated that the only suture present in that form runs at the margin of
the cephalic border, and is similar to that of _Trinucleus_. This matter
will be referred to again when discussing the nature of the eyes in
_Trinucleus_ and _Harpes_.
The relative sizes of the fixed and free cheeks obviously depend on the
position of the facial suture; when this starts on the lateral margin of
the cephalic shield and passes forward to the outer part of the front
margin, the free cheek will be a narrow strip; when, on the other hand,
the suture starts from the posterior margin and runs close to the
glabella, the free cheek will be relatively large and the fixed cheek
narrow. The fixed cheek is small in _Phacops_, _Cheirurus_, and
_Illaenus_; relatively large in _Remopleurides_, _Phillipsia_, and
_Stygina_. It was suggested by M‘Coy[183] that the free cheek represents
the pleura of an anterior segment which has not become fused with the
other cephalic pleurae. The fixed cheek appears to be formed of the
coalesced pleurae of the other cephalic segments, but of those pleurae
the only indication seen in adult specimens is in the neck-ring; in
young specimens of _Olenellus_, however, the presence of other pleurae
is indicated by furrows on the cheeks in front of the neck-furrow.
[Illustration:
FIG. 138.—_Phacops latifrons_, Bronn, × 1. Devonian. Showing large
compound eye. (After Zittel.)
]
A pair of compound =eyes= are present in the majority of Trilobites.
Each eye is situated on the free cheek, at that part of its inner margin
where the facial suture bends to form an angle (Figs. 137, A, _h_, 138).
The position of the eye is consequently determined by the position of
the facial suture; it may be near the glabella or near the lateral
margin of the head, and either as far forward as the first segment of
the glabella or nearly as far back as the neck-furrow. In many
Trilobites the eye is more or less conical, with its summit truncated or
rounded, but in some genera it is ovoid, or crescentic. In _Aeglina_
(Fig. 150, H) the eye is flattened and scarcely raised above the general
level of the cheek. The eye of a Trilobite is oriented so that its
longer axis is parallel or nearly parallel to the axis of the body (Fig.
150, G); but in one case (_Encrinurus intercostatus_) it is placed at
right angles to this axis. The size of the eye varies considerably; it
is largest in _Aeglina_, in which it covers nearly the whole of the free
cheek; it is small in _Acidaspis_ and _Encrinurus_.
Though the eye is always entirely on the free cheek, the adjoining part
of the fixed cheek is raised to form a buttress on which the eye rests;
this buttress, which is known as the “palpebral lobe,” is seen clearly
when the fixed cheek is removed. The eyes of Trilobites are always
sessile; for although in some species, such as _Asaphus cornigerus_, _A.
kowalewskii_, and _Encrinurus punctatus_, they are on the summits of
prominent stalks, yet those stalks are immovable.
Three types of compound eye have been recognised in Trilobites[184]—
holochroal, prismatic, and schizochroal.
[Illustration:
FIG. 139.—Eyes of Trilobites. (After Lindström.) =A=, =B=,
_Sphaerophthalmus alatus_, Ang. Upper Cambrian. Vertical and
horizontal sections, × 100. =C=, _Asaphus fallax_, Dalm. Horizontal
section, × 60. =D=, _Nileus armadillo_, Dalm. Vertical section, ×
60, _a_, prismatic lenses; _b_, cuticle; _c_, part of free cheek.
=E=, _Dalmanites vulgaris_, Salt. Part of eye, × 30. =F=,
_Dalmanites imbricatulus_, Ang. Vertical section of eye, with a part
of the free cheek on the left, × 60. =G=, =H=, _Harpes vittatus_,
Barr. =G=, The two lenses of one eye, × 8; =H=, vertical section of
the same, × 60.
]
1. In the holochroal eye (Fig. 139, A, B) the lenses are globular or
biconvex and close together, so that the cornea is continuous over the
entire eye. Examples of this are seen in _Bronteus_ and
_Sphaerophthalmus_.
2. In the prismatic type (Fig. 139, C, D) the lenses are prismatic and
plano-convex, and the entire surface of the eye is covered by a smooth
cuticle. The lenses are close together and usually hexagonal, but
occasionally rhombic or square. Near the margin of the eye the lenses
may become irregular, giving rise to a border in which the prismatic
structure is more or less indistinct. The prismatic type of eye is found
in the genera _Asaphus_, _Nileus_, _Illaenus_, etc.
3. The schizochroal eye occurs only in the family Phacopidae (Fig. 139,
E, F). The lenses are biconvex and are separated by portions of the
cephalic shield, so that each lens appears to rest in a separate socket,
and the cornea is not continuous for the entire eye. The lenses are
circular in outline, but owing to the upward and inward growth of the
interstitial test they may appear, on the surface, to be hexagonal. The
diameter of a lens may be as much as 0·5 mm. The crystalline cones have
not been preserved. In specimens of _Phacops rana_, in which the inner
face of the lens is more convex than the outer, J. M. Clarke[185] has
obtained evidence of a posterior spheroidal cavity in addition to the
anterior corneal cavity. The complete separation of the lenses in this
type of eye has led to the suggestion that the schizochroal eye is an
aggregate rather than a compound eye. But the difference between this
and the holochroal eye is probably less than appears at first sight if
the statement made by Clarke is confirmed, namely, that in young
specimens of _Calymene senaria_ the lenses are relatively large and
similar to those of _Phacops_, whereas in the adult the eye is
holochroal.
These three types of eye, according to Lindström, have appeared
successively in chronological order: the prismatic occurring first in
the _Olenus_ beds (Upper Cambrian), the holochroal first in the
_Ceratopyge_ Limestone (Uppermost Cambrian), and the schizochroal first
in the Ordovician. The number of lenses in the eye varies greatly. For
example, in _Trimerocephalus volborthi_ there are 14 only, whilst in
_Remopleurides radians_ there are as many as 15,000. Even in different
species of the same genus there may be considerable differences. Thus
_Bronteus brongniarti_ possesses 1000, _B. palifer_ 4000, lenses in each
eye. The number increases from the young up to the adult, but decreases
in old age. The lenses are usually arranged in alternating rows. In
Trilobites with a conical eye the outer segment of the cone bears the
visual surface. It has been stated that the eyes of Trilobites resemble
those of Isopods,[186] but close comparison is difficult to make, since
in Trilobites no part of the eye beneath the lenses is preserved. In
some genera a threadlike ridge, called the “eye-line,” passes from the
glabella, generally from the front segment, to the eye, where it often
ends in the palpebral lobe; this eye-line is found in nearly all genera
which are confined to the Cambrian period, and persists in a few of
later date, as for example in _Triarthrus_, _Euloma_, and some species
of _Calymene_ from the Ordovician; in _Arethusina_ and _Acidaspis_ from
the Silurian; and in _Harpes_ from the Devonian (Fig. 150, A).
[Illustration:
FIG. 140.—Trinucleidae. =A=, _Orometopus elatifrons_, Ang. × 5.
Restoration based on specimens from the Upper Cambrian (Tremadoc) of
Shineton, Shropshire. =B=, _Trinucleus bucklandi_, Barr. Ordovician,
Bohemia. A complete but not fully-grown individual showing eyes.
Natural size. (After Barrande.) =C=, _Ampyx rouaulti_, Barr. × 3.
Ordovician, Bohemia. (After Barrande.)
]
In _Harpes_ and in some species of _Trinucleus_ eyes are present, but
have been stated to be of a different type. They are described as simple
eyes, and have been compared with ocelli; they are never found in
Trilobites which possess the compound eyes described above. In _Harpes_
(Fig. 150, A) the eye is near the middle of the cheek, in the position
where compound eyes occur in other genera; it appears to consist of two
or three granules or tubercles which are really lenses, and is connected
with the front of the glabella by an eye-line. No facial suture can be
seen, consequently the whole of the cheek is stated to be the fixed
cheek.[187] In _Trinucleus_ (Fig. 140, B) a single tubercle is found on
the middle of the cheek in the young of some species, and is sometimes
connected with the glabella by an eye-line; the latter disappears before
the adult state is reached, and in some species the tubercle also
disappears, but in others (such as _T. seticornis_, _T. bucklandi_) it
persists in the adult individuals.
From the lateral position of these eyes they can hardly be compared with
the median simple eye of other Crustacea. In _Harpes_ it is more
probable that, as suggested by J. M. Clarke, they are schizochroal eyes
imperfectly developed. Their structure (Fig. 139, G, H) is somewhat
similar to that of schizochroal eyes, and moreover, in one species, _H.
macrocephalus_,[188] there are, in addition to the three main tubercles,
other smaller tubercles in regular rows. Further, the eye-line occupies
the same position as in other Trilobites which have undoubted compound
eyes. The absence of a facial suture cannot be taken as evidence against
these eyes being of the ordinary type, since in some species of
_Acidaspis_ (e.g. _A. verneuili_, _A. vesiculosa_) which possess
compound eyes there is, in consequence of the coalescence of the fixed
and free cheeks, no suture.
In some species of _Trinucleus_ (Fig. 140, B) the simple eye is found in
the same position as the eye in _Harpes_, and if, as some writers have
maintained, there is evidence of the existence of a suture in that
genus, then there is no reason for regarding the eye as other than a
degenerate form of compound eye. The probability of its being such is
supported by the existence of a compound eye in a similar position in
the allied form _Orometopus_ (Fig. 140, A) which possesses a facial
suture.
In some species of _Trinucleus_ (Fig. 140, B) and _Ampyx_ there is a
small median tubercle on the front part of the glabella, which from its
position may be a simple unpaired eye, but its structure appears to be
unknown.
Some Trilobites possess no eyes. Well-known examples of such are
_Agnostus_, _Microdiscus_, _Ampyx_, _Conocoryphe_, and some species of
_Illaenus_ and _Trinucleus_; such blind Trilobites are almost confined
to the Cambrian and Ordovician periods. All the forms of later periods,
with the exception of a species of _Ampyx_, and possibly one or two
other species, possess eyes. In addition to those undoubtedly blind
forms Lindström considers that most of the Olenidae and Paradoxidae were
without eyes. Many of the members of these families possess a lobe
closely resembling a palpebral lobe, and a corresponding excavation in
the free cheek; such forms have been generally regarded as possessing
eyes; and the absence of any indication of lenses in those cases, on
which Lindström lays stress, has been explained by the comparatively
imperfect preservation of these early Trilobites. The development of the
supposed eye-lobe in some of the Paradoxidae and Olenidae differs from
that of the eyes in other families of Trilobites. In the latter the eye
appears first at the margin of the head and always in connexion with the
facial suture. But in _Olenellus_, in which there is said to be no
facial suture, development shows that the crescentic eye-like lobe (Fig.
145, E) is really of the nature of a pleura coming off from the base of
the first segment of the glabella. In _Paradoxides_, which resembles
_Olenellus_ in many respects, a facial suture is present and forms the
outer boundary of the eye-like lobe, but it is developed subsequently to
the appearance of the latter, which seems to be similar to that of
_Olenellus_. In some genera of the Olenidae the eye-line, which comes
off from the first segment of the glabella, ends in some cases in a
swelling or knob which has hitherto been regarded as a palpebral lobe,
but according to Lindström’s view no trace of an eye has been found in
connexion with that lobe, nor is there any space between the lobe and
the free cheek in which the eye could have occurred. If this view is
correct it follows that the majority of the Cambrian Trilobites were
blind. The earliest genus with eyes would then be _Eurycare_ found in
the _Olenus_ beds of the Upper Cambrian. _Sphaerophthalmus_ and
_Ctenopyge_, found in the higher beds of the Cambrian, also possessed
eyes, but _Olenus_ and _Parabolina_ were probably blind.
On the ventral surface of the head there is a flat rim around the margin
(Fig. 137, B, _b_); this rim or “doublure” is the reflexed border of the
cephalic shield. In many Trilobites its median part in front is cut off
by sutures so as to form a separate plate (_e_); such is the case when
the two facial sutures (_c_, _c′_) cut the anterior margin of the
cephalic shield and are continued across the doublure, where they are
joined by a transverse or rostral suture (_d_) just below the margin.
When, however, as in _Phacops_ and _Remopleurides_, the two facial
sutures unite on the dorsal surface, in front of the glabella, the
median part of the doublure is not separated from the lateral parts, or
from the dorsal part of the cephalic shield.
[Illustration:
FIG. 141.—=A=, Hypostome of _Bronteus polyactin_, Ang. showing
maculae, × 4. =B=, Left macula of _Bronteus irradians_, Lindst. ×
12. (After Lindström.)
]
The “labrum” or “hypostome” is attached to the doublure in front (Fig.
137, B, _a_); it is commonly an oval or shield-shaped plate, but is
occasionally nearly square. Its surface is sometimes divided into two or
three areas by shallow transverse grooves (Fig. 141, A), Just behind the
middle of the hypostome, or when transverse grooves are present either
in or near the anterior groove, there are often found a pair of small
patches or “maculae” which are more or less oval or elliptical in
outline (Fig. 141). The maculae may be (1) surrounded by a raised
border, or (2) in the form of pits, or (3) raised like tubercles. In
some cases the entire surface of a macula is smooth and glossy; in
others either the whole or a part is covered with granules, and in the
latter case the granules may be limited to the internal third (Fig. 141,
B) or to the central portion. Sections of a macula show that the
granules are really globular lenses similar to those of the compound
eyes on the dorsal surface of the head. Some of the maculae which are
without lenses show no structure, but in others there is a spongy or
irregularly polyhedric structure with prisms, resembling the marginal
zone of the prismatic eyes of some genera. There seems no doubt that the
maculae with lenses are visual organs, and those without are degenerate
eyes. They occur in some genera which, according to Lindström, are
without eyes on the dorsal surface. Maculae do not appear to be present
in other Crustacea, but they have been compared with a median organ,
found just in front of the hypostome in _Branchipus_.[189] Maculae, have
so far been found in 136 species of Trilobites belonging to 39 genera
ranging from Lower Cambrian to Carboniferous.
A “metastoma” or lower lip plate (Fig. 142, _Ep_) is found just behind
the hypostome in _Triarthrus_, but has not been noticed in any other
genus. Between the hypostome and the metastoma lies the mouth.
The segments of the =thorax= are free, and their number varies from two
in _Agnostus_ (Fig. 146) to twenty-six in _Harpes_ (Fig. 150, A). In the
Trilobites confined to the Cambrian period the number (except in the
Agnostidae) is usually larger than in the genera found in the Ordovician
and later periods. Owing to the free thoracic segments many Trilobites
were able to curl up somewhat after the manner of a Wood-louse (Figs.
137, D, 138). The axial part of each thoracic segment is more or less
considerably arched. Usually it consists of three parts: (i.) the
largest part (Fig. 137, C, _a_), called the ring, is band-like in form,
and is always visible whether the Trilobite is extended or coiled up;
(ii.) in front of the ring is a depressed, groove-like part (Fig. 137,
C, _b_) separating it from (iii.) the articular portion (_c_) which is
convex in front and extends beneath the ring of the preceding segment;
this part is only visible when the Trilobite is coiled up or when the
segments are separated. In some few genera the axial part consists of a
simple arched band without either a groove or a specially modified
articular portion. The pleurae (Fig. 137, A, _l_, C, _d-f_) are fixed
firmly to the axis, and have the form of narrow bands with the ends
rounded, obtuse, pointed, or spinose. In a few cases the pleurae have a
plain surface; but usually they possess either a ridge or a groove (Fig.
137, C, _g_); the former is generally parallel to the margins of the
pleura, the latter is generally oblique, being inclined backwards from
the axis. Sometimes in front of the ridge there is a small groove. On
the ventral surface each pleura shows, at its outer extremity, a
reflexed margin or doublure. At some distance from the axis the pleurae
are bent downwards and backwards. The point where this bend occurs is
called the “fulcrum” (_e_); it divides the pleura into an internal and
an external part: the internal part (_d-e_) is flat or slightly convex,
and just touches the front and back margins of the adjacent pleurae; the
external part (_e-f_) may be (i.) narrower than the internal part, so
that it is separated from the previous and succeeding pleurae; such
occurs principally in pleurae with ridges, as in _Cheirurus_ and
_Bronteus_; or (ii.) it may be in the form of a long cylindrical
process, as in many species of _Acidaspis_; or (iii.) the external part
may be of the same width, either throughout or in part, as the internal
part, and may overlap the next pleura behind; this type is found
principally in pleurae with a groove such as in _Phacops_, _Calymene_,
_Sao_, _Asaphus_, _Ellipsocephalus_.
In some Trilobites there is beyond the fulcrum a smooth, flat,
triangular part at the front margin of the pleura; this part is known as
the “facet,” and forms a surface articulating with the preceding segment
which overlaps it.
In the remarkable form _Deiphon_ (Fig. 151, E) the pleurae are separate
throughout their entire length.
In some Trilobites broad and narrow forms of the same species occur—the
difference being seen especially in the axis. The former are regarded as
females, the latter as males.[190]
The segments of the =abdomen= or =pygidium= (Fig. 137, A, 3) are similar
to those of the thorax, except that they are fused together. In a few
forms, such as _Illaenus_ (Fig. 150, F) and _Bumastus_, the fusion is so
complete that no trace of segmentation can be seen on the dorsal
surface. Usually, however, the segments are easily distinguishable; the
number seen on the axis is commonly greater than on the lateral parts of
the pygidium; this difference is particularly well shown in
_Encrinurus_. In Trilobites which have grooved pleurae the conspicuous
grooves seen on the lateral parts of the pygidium are the grooves of the
pleurae, the sutures _between_ the pleurae being less distinct. The
shape of the pygidium may be semicircular, a segment of a circle,
trapezoidal, triangular, semi-parabolic, etc.; its size varies
considerably; in the Cambrian forms it is usually small, but in the
Trilobites of later periods it becomes relatively larger. The number of
segments in the pygidium varies from two to twenty-eight. The axis of
the pygidium tapers more rapidly than that of the thorax; sometimes it
reaches quite to the posterior end of the body, but is commonly shorter
than the pygidium; in _Bronteus_ it is extremely short, and the grooves
on the lateral parts of the pygidium radiate from it in a fan-like
manner. Occasionally, as in _Bumastus_, the axis cannot be distinguished
from the lateral parts. In a few early Trilobites (_Olenellus_,
_Holmia_, Fig. 148, _Paradoxides_, Fig. 147) the lateral parts of the
pygidium are very small. In some genera, such as _Asaphus_, the marginal
part of the pygidium forms a flattened or concave border. The margin may
be entire or produced into spines, and sometimes (Fig. 151, C) a caudal
spine comes off from the end of the axis. On the ventral surface of the
pygidium there is a marginal rim similar to the doublure of the cephalic
shield. The anus is on the ventral surface of the last segment of the
pygidium.
Although Trilobites are often found in abundance and in an excellent
state of preservation, it is only in very rare cases that anything is
seen of the ventral surface except the hypostome and the reflexed
borders of the cephalic shield, of the thoracic segments, and of the
pygidium. The usual absence of =appendages= is probably due to their
tenuity. Billings, in 1870, first obtained clear evidence of the
presence of pairs of appendages, in _Asaphus platycephalus_. Soon
afterwards Walcott[191] showed their existence in American specimens of
_Asaphus megistos_, _Calymene senaria_, and _Cheirurus pleurexacanthus_.
In the two latter species the appendages were found by cutting sections
of curled-up specimens obtained from the Trenton Limestone; 2200
examples were sliced, of which 270 showed evidence of the existence of
appendages. They were seen to be present on the head, thorax, and
pygidium; a ventral uncalcified cuticle with transverse arches was also
found. By means of sections of curled-up specimens it was difficult to
determine satisfactorily the form and position of the appendages.
Subsequently extended specimens of _Triarthrus_ (Fig. 142) and
_Trinucleus_, showing the ventral surface and appendages clearly, were
discovered in the Utica Slate (Ordovician) near Rome, New York. A full
account of the appendages in those specimens has been given by
Beecher.[192]
[Illustration:
FIG. 142.—_Triarthrus becki_, Green, × 2½. Utica Slate (Ordovician),
near Rome, New York. =A=, Ventral surface with appendages; _Ep_,
metastome; _Hy_, hypostome. =B=, second thoracic appendage; _en_,
endopodite; _ex_, exopodite, × 12. (After Beecher.)
]
In _Triarthrus_ each segment, except the anal, bears a pair of
appendages, all of which, except the first, are biramous. There are five
pairs of cephalic appendages; the first pair are attached at each side
of the hypostome, and have the structure of antennae, each consisting of
a single flagellum formed of short conical joints. The other cephalic
appendages increase in size successively. At present the second and
third pairs are not satisfactorily known, but appear to have been
similar to the fourth and fifth pairs. The second pair is attached at
the level of the posterior end of the hypostome. The fourth and fifth
pairs have large, triangular coxopodites which served as gnathobases,
their inner edges being denticulate; the endopodites consist of stout
joints; the exopodites are slender, and bear setae which are often
arranged in a fan-like manner.
The first pair of appendages appear to be antennules, whilst the second
pair probably represent the antennae, the third pair the mandibles, and
the fourth and fifth pairs the maxillae of other Crustacea. The
appendages of the thorax and pygidium do not differ essentially from the
two posterior cephalic appendages. Those on the anterior part of the
thorax are the longest; the others gradually decrease in size in passing
posteriorly. Each thoracic leg (Fig. 142, B) consists of a short
coxopodite with an inward cylindrical prolongation forming a gnathobase
which is best developed on the anterior legs; the endopodite and
exopodite are long and nearly equal; the former consists of six joints
tapering gradually to the end; the latter, of a long proximal joint with
a denticulate edge and a distal part of ten or more joints, and it bears
a row of setae along the whole of the posterior edge.
The anterior appendages of the pygidium differ but little from the
posterior thoracic legs; but the phyllopodous character, which appears
in the latter, becomes more distinct in the appendages of the pygidium,
especially those near its posterior end, and is due to the broad, flat,
laminar joints of the endopodite.
The more striking features of the appendages of _Triarthrus_ are the
small amount of differentiation which they show in different parts of
the body, especially the want of specialisation in the cephalic region;
the distinctly biramous character of all except the first pair; and the
presence of one pair of functional antennae only, and the occurrence of
thoracic gnathobases.
In _Trinucleus_ the appendages are not so well known, but they are
considerably shorter than in _Triarthrus_.
In the Palaeozoic rocks of Bohemia, where Trilobites are very perfectly
preserved, Barrande[193] discovered the larval forms of several species,
and in some cases was able to trace out the =development= very
completely, but in others the earliest stages were not found. In the
strata in which Trilobites occur Barrande found minute spheroidal
bodies, usually of a black colour, and only a little smaller than the
youngest larval stages; those bodies are probably the eggs of
Trilobites. Since the publication of Barrande’s work the development of
some species found in North America has been studied by Ford, Matthew,
Walcott, and Beecher. But even now the development is known in only a
very small proportion of the total number of genera of Trilobites. The
early larval form (Fig. 143, A) is similar in general character in the
various species in which it has been found. It is circular or ovoid in
outline, with a length of from 0·4 to 1 mm., and consists of a large
cephalic and a small pygidial portion; the axis is distinct and usually
shows more or less clear indications of five cephalic segments; the
eyes, when present, are found at or near the front margin, and the free
cheeks, if visible at all on the dorsal surface, are narrow. For this
early larval form Beecher has proposed the name “protaspis”; he regards
it as the representative of the Nauplius of other Crustacea, but that
view is not accepted by Professor J. S. Kingsley.[194]
The general changes which occur in the course of development are:
modifications in the shape and relative size of the glabella, and of the
number and depth of the glabella-furrows; the growth of the free cheeks
and the consequent inward movement of the facial sutures and eyes; the
introduction of and gradual increase in number of the thoracic segments,
and the relative decrease in size of the head.
[Illustration:
FIG. 143.—Development of _Sao hirsuta_, Barr. Cambrian. =A=,
Protaspis; =B-F=, later stages; =G=, adult. The small outlines below
each figure show the actual size of each specimen. (After Barrande.)
]
_Sao hirsuta_ is a species found in the Cambrian, the development of
which was fully described by Barrande. Its earliest protaspis stage
(Fig. 143, A) is circular in outline; the glabella expands in front and
reaches the anterior margin; the pygidial region is not distinctly
separated from the cephalic region; segmentation is indicated in the
former, and the neck-ring is present in the latter; the eye-line is seen
on each side of the glabella near the anterior margin. In a later stage
(Fig. 143, C) the segmentation of the glabella becomes more distinct,
indicating the existence of five cephalic segments, and the facial
suture appears near the margin limiting a very narrow free cheek.
Subsequently (Fig. 143, D-F) the thoracic segments develop, and increase
in number until the adult stage (G) is reached; also the eyes appear at
the margin of the cephalic shield, and gradually move inwards, and the
glabella becomes narrower and rounded in front, and ceases to reach the
anterior margin. In this species the eye-line is present in the adult.
[Illustration:
FIG. 144.—_Triarthrus becki_, Green. Ordovician. =A=, =B=, Two
successive stages of the protaspis, × 45. (After Beecher.)
]
In the protaspis of _Triarthrus_ (Fig. 144), found in the Ordovician,
the glabella does not reach the front margin nor expand in front as it
does in _Sao_; an eye-line is present, but disappears before the adult
stage is reached.
[Illustration:
FIG. 145.—Larval stages of Trilobites. =A-D=, _Dalmanites socialis_,
Barr. Ordovician, Bohemia. The small figures below show the natural
size of each specimen. (After Barrande.) =E=, _Mesonacis
asaphoides_, Emmons, × 10. Lower Cambrian, North America. (After
Walcott.) =F=, _Acidaspis tuberculata_, Conrad, × 20. Lower
Helderberg Group (Lower Devonian or Upper Silurian), Albany County.
(After Beecher.)
]
_Dalmanites_ (Fig. 151, C) is a more advanced type than _Sao_ and
_Triarthrus_, and is found in later deposits. In the earliest stage
(Fig. 145, A) the head and pygidium are quite distinct, and there is no
eye-line present at this or any stage in development, but large ovoid
eyes are found on the front margin, and have their long axes placed
transversely to the axis of the body; the glabella is strongly segmented
and is rounded in front. In later stages (C, D) the pygidium increases
in size relatively, and the thoracic segments are successively
introduced; the facial sutures and free cheeks appear on the dorsal
surface, and as the free cheeks grow the eyes move inwards and
backwards, and gradually swing round until their long axes become
parallel with the axis of the body.
The larval form of _Acidaspis_ (Fig. 145, F) is of interest since even
in the earliest stage it shows the spiny character which forms such a
striking feature of the adult (Fig. 151, F).
Before the discovery of the ventral surface of Trilobites it was thought
by some zoologists that their =affinities= were with the Xiphosura
rather than with the Crustacea. But the presence of antennae, and of
five pairs of cephalic appendages; the biramous thoracic and pygidial
appendages, the hypostome, and the character of the larval form, as well
as the absence of a genital operculum, separate the Trilobites from the
Xiphosura and connect them with the Crustacea.
The position of the Trilobites in the Crustacea is, however, difficult
to determine. Already in the Cambrian period, at least five main groups
of the Crustacea were clearly differentiated, namely, the Phyllopoda,
Ostracoda, Cirripedia, Trilobita, and Leptostraca (Phyllocarida), and
probably also the Copepoda, but of the last no remains have been
preserved as fossils. Palaeontology, therefore, furnishes no connecting
links between any two of these orders.
The Crustacea to which the Trilobites show some resemblance are the
families Apodidae and Branchipodidae of the Order Phyllopoda (see pp.
19–36). The Trilobita agree with those families in having a large but
variable number of trunk-segments, in the possession of a large labrum
(hypostome), and in the occurrence of gnathobases on the thoracic
appendages; also the foliation of some of the trunk-appendages is
somewhat similar. The points of difference, however, are considerable;
thus the cephalic appendages are much more specialised in the Apodidae
and Branchipodidae than in the Trilobita; in the latter all, with the
exception of the antennae, are distinctly biramous, and whilst the basal
joints were masticatory the distal parts appear to have been locomotor
organs. The appendages of the trunk also differ considerably; in the
Trilobita all are clearly biramous, those of the thorax having a
schizopodal form. In the possession of a single pair of antennae the
Trilobita differ from other Crustacea; but in some forms of _Apus_ the
second pair of antennae may be rudimentary or even absent.
There are still other features which characterise the Trilobita: thus
the eyes are borne on free cheeks, and differ in structure from those of
Phyllopods. The broad pygidium formed of fused segments and without
terminal fulcra is quite unlike the slender-jointed abdomen of _Apus_
and _Branchipus_; and whilst in the Trilobites all the segments bear
appendages, in the Phyllopods some, at any rate, of the posterior
segments are devoid of appendages. The distinct division of the body
into an axial and pleural region is not seen in Phyllopods, and is
probably a character of some importance, since it occurs in the great
majority of Trilobites, including all the early forms.
The existence of some relationship between the Trilobita and the
Leptostraca (Phyllocarida) has been maintained by Professor G. H.
Carpenter.[195] He points out that some of the earliest Trilobites, such
as _Holmia kjerulfi_ (Fig. 148), possess nearly the same number of
segments as _Nebalia_ (Fig. 76, p. 111), and that in the latter genus
the cephalic appendages, especially the mandibles and maxillae, are less
specialised than in _Apus_, and consequently differ less from those of
Trilobites than do the appendages of the Apodidae. Further, in another
genus of the Leptostraca, _Paranebalia_, the biramous thoracic legs, in
which both endopodite and exopodite are elongate, approach those of
Trilobites more nearly than do the thoracic legs of _Apus_.
The view[196] that some connexion may exist between the Isopoda and the
Trilobita seems to have been based on the similar dorso-ventral
flattening of the body, its division into three regions—head, thorax,
and abdomen—and the presence of sessile eyes. Beyond this it is
difficult to find any resemblance; whilst the differences, such as the
variable number of thoracic segments and their biramous appendages in
Trilobites, are important.
At present, then, we can only conclude that the Trilobita are more
primitive than any other Crustacea, and that their resemblance to some
of the Phyllopoda is sufficient to make it probable that they had some
ancestral connexion;[197] the possibility of such a relationship
receives some support from the presence in the Lower Cambrian rocks of
_Protocaris_, a genus of the Phyllopoda which resembles _Apus_.[198] The
primitive characters of Trilobites are the variable and often large
number of segments in the thorax and pygidium; the presence of a pair of
appendages on every segment except the anal; the biramous form of all
except the first pair of appendages; and the lack of specialisation
shown by the appendages, especially those of the head.
The =classification= of Trilobites is due largely to the work of
Barrande and Salter, and the families defined by those authors have
been, in the main, generally adopted. But the phylogenetic relationship
of the families has still, to a large extent, to be established.
Salter[199] arranged the families in four groups, but did not claim that
that classification was entirely natural. His groups with the families
included in each are:—
1. _Agnostini._ Without eyes or facial suture. Agnostidae.
2. _Ampycini._ Facial sutures obscure, or submarginal, or absent. Eyes
often absent. Trinucleidae.
3. _Asaphini._ Facial sutures ending on the posterior margin.
Acidaspidae, Lichadidae, Harpedidae, Calymenidae, Paradoxidae,
Conocephalidae, Olenidae, Asaphidae, Bronteidae, and Proëtidae.
4. _Phacopini._ Facial sutures ending on the lateral margins. Eyes well
developed. Phacopidae, Cheiruridae, and Encrinuridae.
A modification of Salter’s classification has been brought forward by
Beecher[200] who divides the Trilobita into three main groups:—
1. _Hypoparia._ Facial sutures at or near the margin, or ventral.
Compound eyes absent. This is equivalent to Salter’s Agnostini and
Ampycini with the addition of the Harpedidae.
2. _Opisthoparia._ Facial sutures extending from the posterior margin to
the front margin, but occasionally uniting in front of the glabella.
Eyes holochroal or prismatic, but sometimes absent. This comprises the
same families as Salter’s Asaphini with the exclusion of the Harpedidae
and Calymenidae.
3. _Proparia._ Facial sutures extending from the lateral margins, and
either cutting the anterior margin or uniting in front of the glabella.
Eyes holochroal or schizochroal; occasionally absent. This is equivalent
to Salter’s Phacopini with the addition of the Calymenidae.
In each of the groups proposed Beecher regards as the more primitive
forms those which possess characters similar to those of the early
larval stages, such as narrow free cheeks, the absence of compound eyes,
and a glabella which is broad in front and reaches the anterior margin
of the head.
The modifications introduced by Beecher can scarcely be regarded as
making Salter’s classification more natural. For instance, the
Agnostidae differ so much from all other families that, at present,
there is no evidence to show that they have any close phylogenetic
relationship with the Trinucleidae and Harpedidae. Further, the
Calymenidae, which Salter recognised as related to the Olenidae, have
been shown by the careful work of Professor Pompeckj[201] to have
descended from the latter family, and to have no genetic connexion with
the Phacopidae with which they are grouped by Beecher. Then in the
Trinucleidae the earliest genus, _Orometopus_[202] (Fig. 140, A),
possesses compound eyes and facial sutures which begin at the posterior
margin and unite in front of the glabella; so that, according to
Beecher’s classification, that genus would belong to the Opisthoparia,
whereas the later genera (_Trinucleus_, etc.) of the same family would
be placed in the Hypoparia. At present, therefore, the only
classification of Trilobites which can be adopted is a division into
families, of which a short account is given below.
[Illustration:
FIG. 146.—_Agnostus integer_, Beyr., × 8. Cambrian. (After Barrande.)
]
=Fam. 1. Agnostidae= (Fig. 146).—Small Trilobites, in which the head and
pygidium are of nearly the same size and shape. The thorax is shorter
than the head or pygidium, and consists of from two to four segments
with grooved pleurae. The length and width of the head are commonly
nearly equal, but sometimes the length is greater. Eyes are absent.
Facial sutures appear to be absent, but are stated by Beecher to be at
the margin of the cephalic shield. From the absence of eyes, the
probable absence of facial sutures, the few or indistinct furrows on the
glabella, and the smaller number of thoracic segments, the Agnostidae
appear to be degenerate forms. _Microdiscus_ is apparently less modified
than _Agnostus_, on account of the larger number of thoracic segments,
the more distinct segmentation of the pygidium, and, in some species,
the larger number of furrows on the glabella. Cambrian and Ordovician.
Genera: _Agnostus_, _Microdiscus_.
=Fam. 2. Shumardiidae.=—The body is very small and oval. The cephalic
shield is nearly semicircular and very convex, with a broad glabella
which expands in front, and in which the furrows, except the
neck-furrow, are indistinct. The facial suture is marginal and eyes are
absent. There are six thoracic segments with ridged pleurae; the axis is
broader than the pleurae. The pygidium is large, and is formed of about
four segments similar to those of the thorax. Upper Cambrian and
Ordovician. Genus: _Shumardia_.
=Fam. 3. Trinucleidae= (Fig. 140).—The head is large and has a flat
border (except in _Ampyx_), and long genal spines. In the earliest genus
(_Orometopus_) the facial sutures start from the posterior margin (near
the genal angle) and pass obliquely inwards to the compound eye, from
whence they continue forward and unite in front of the glabella. In
_Ampyx_ the suture starts from just within the genal angle and passes to
the front border, cutting off a narrow free cheek; eyes are absent. In
most specimens of _Trinucleus_ no sutures are seen, but some examples
show indications of what may be a facial suture (see p. 226), and a
suture is sometimes found at the margin of the cephalic border; eyes may
occur (see p. 230). The thorax consists of from five to eight segments,
with grooved pleurae. The pygidium is triangular. Principally
Ordovician. Genera: _Orometopus_ (Upper Cambrian), _Ampyx_,
_Trinucleus_, _Dionide_.
=Fam. 4. Harpedidae= (Figs. 139, G, H; 150, A).—The head is large and
has a broad, flat border which is finely punctate, and extends backwards
on each side in the form of a horn-like projection nearly as far as the
posterior end of the thorax. The glabella is convex and does not reach
the front margin. The cheeks are less convex than the glabella, and bear
eyes which usually consist of two or three lenses. An eye-line connects
the eye with the anterior part of the glabella. A suture is stated to
occur at the external margin of the flat border. The thorax consists of
from twenty-five to twenty-nine segments; its axis is narrow, and the
pleurae are long and grooved. The pygidium is very small, and consists
of three or four segments. Ordovician to Devonian. Genus: _Harpes_.
[Illustration:
FIG. 147.—_Paradoxides bohemicus_, Barr. × ½. Middle Cambrian. (After
Zittel.)
]
[Illustration:
FIG. 148.—_Holmia kjerulfi_, Linnars. × 1. Lower Cambrian. (After
Holm.)
]
=Fam. 5. Paradoxidae= (Figs. 147, 148, 149).—The cephalic shield is
large, and bears long genal spines. The glabella is more or less swollen
in front. The facial sutures appear to be absent in some genera, and
when present extend from the posterior to the anterior margin. The
palpebral lobes are long, and often more or less semicircular or
kidney-shaped. The thorax is long, and consists of from sixteen to
twenty segments with their pleurae produced into spines. The pygidium is
very small, and plate-like, or sometimes in the form of a long spine.
Cambrian. Genera: _Olenellus_, _Holmia_, _Mesonacis_, _Olenelloides_,
_Paradoxides_, _Zacanthoides_, _Centropleura_ (_Anopolenus_).
_Remopleurides_ (Fig. 150, D) from the Ordovician is usually included in
the Paradoxidae, but probably belongs to a separate family.
[Illustration:
FIG. 149.—_Clenelloides armatus_, Peach. Lower Cambrian, × 3. (After
Peach.)
]
=Fam. 6. Conocephalidae (Conocoryphidae)= (Fig. 150, E).—The cephalic
shield is semicircular, and larger than the pygidium. The glabella
narrows in front. The facial suture passes from near the genal angle on
the posterior border to the antero-lateral margin, and limits a large
fixed cheek and a narrow free cheek. Eyes are absent or rudimentary, but
an eye-line is usually present. The thorax consists of from fourteen to
seventeen segments with grooved pleurae, which may be pointed, but are
not usually produced into spines. The pygidium is small, and formed of
few segments. Cambrian. Genera: _Conocoryphe_, _Atops_, _Ctenocephalus_,
_Bathynotus_.
=Fam. 7. Olenidae= (Figs. 142, 143; 150, B, C).—The cephalic shield is
larger than the pygidium. The glabella is either rectangular or
parabolic. The facial suture passes from the posterior to the anterior
margin. The palpebral lobes are of moderate or rather large size, and
are connected by an eye-line with the front part of the glabella. The
thorax includes from eleven (occasionally fewer) to eighteen segments
with grooved pleurae. The pygidium is usually small, with from two to
eight segments. Principally Cambrian. Genera: _Ptychoparia_, _Angelina_,
_Solenopleura_, _Sao_, _Agraulos_ (_Arionellus_), _Ellipsocephalus_,
_Protolenus_, _Olenus_, _Peltura_, _Acerocare_, _Eurycare_, _Ctenopyge_,
_Leptoplastus_, _Triarthrus_, _Parabolina_, _Sphaerophthalmus_,
_Parabolinella_, _Ceratopyge_ (position doubtful). _Dikelocephalus_ is
usually placed in the Olenidae, but perhaps belongs to a distinct
family.
=Fam. 8. Calymenidae= (Figs. 136, 137).—The glabella is broadest behind.
The facial suture starts at or near the genal angle—sometimes on the
posterior border just inside the angle, sometimes on the lateral border
just in front of the angle; the suture may be continuous with the other
suture in front of the glabella, or may cut the anterior margin, beneath
which it is connected with the other suture by means of a transverse
suture (Fig. 137, B, D). The eyes are rather small. The thorax consists
of thirteen segments with grooved pleurae; the pygidium of from six to
fourteen segments. Ordovician to Devonian. Genera: _Calymene_,
_Synhomalonotus_, _Homalonotus_.
[Illustration:
FIG. 150.—=A=, _Harpes ungula_, Sternb., Ordovician. =B=,
_Ellipsocephalus hoffi_, Scloth., Cambrian. =C=, _Olenus truncatus_,
Brünn., Cambrian. (After Angelin.) =D=, _Remopleurides radians_,
Barr., Ordovician. =E=, _Conocoryphe sulzeri_, Barr., Cambrian. =F=,
_Illaenus dalmanni_, Volb., Ordovician. =G=, _Proëtus bohemicus_,
Corda, Silurian, × 1½. =H=, _Aeglina prisca_, Barr., Ordovician, ×
3. =I=, _Phacops sternbergi_, Barr., Devonian. (=A=, =D=, =E=, =G=,
=H=, =I=, after Barrande; =B=, =F=, from Zittel; natural size except
=G=, =H=.)
]
=Fam. 9. Asaphidae= (Fig. 150, F).—The body is oval and commonly rather
large. The cephalic shield is large, with its glabella often
indistinctly limited and the glabella-furrows often obscure. The facial
suture starts from the posterior margin and usually cuts the anterior
margin, but is sometimes continued in front of the glabella. The
relative size of the fixed and free cheeks varies greatly. The eyes are
of variable size. The thorax consists of eight or ten (sometimes fewer)
segments; the pleurae are generally grooved, but sometimes plane. The
pygidium is large, often being similar in form and size to the head; it
consists of numerous segments which, however, may be indistinctly shown;
the axis in some forms is obsolete. Upper Cambrian (Tremadoc) to
Silurian; common in the Ordovician. Genera: _Asaphus_ (sub-genera,
_Megalaspis_, _Asaphellus_, _Symphysurus_, etc.), _Ogygia_, _Barrandia_,
_Niobe_, _Nileus_, _Illaenus_, _Bumastus_, _Stygina_. _Aeglina_ (Fig.
150, H) is usually placed in this family, but its systematic position is
doubtful.
=Fam. 10. Bronteidae.=—The general form is similar to that of the
Asaphidae. The glabella broadens rapidly in front, and is marked with
furrows on each side, which are usually short, and may be indistinct.
The facial suture passes from the posterior margin to the crescentic eye
which is situated rather near the posterior border, and from thence to
the anterior margin. There are ten thoracic segments with ridged
pleurae. The pygidium is longer than the head, and has a very short
axis, from which the furrows on the pleural part radiate. Ordovician to
Devonian. Genus: _Bronteus_.
=Fam. 11. Phacopidae= (Figs. 138; 150, I; 151, C).—The head and pygidium
are of about the same size. The glabella is distinctly limited, and
wider in front than behind, with a neck-furrow and three other furrows,
of which some of the anterior may be indistinct or obsolete. The eyes
are schizochroal and usually large. The facial suture begins at the
lateral margin and unites with the suture of the other side in front of
the glabella. There are eleven thoracic segments with grooved pleurae.
The pygidium is usually large, with a distinct axis and many segments.
Ordovician to Devonian. Genera: _Phacops_, _Trimerocephalus_, _Acaste_,
_Pterygometopus_, _Chasmops_, _Dalmanites_, _Cryphaeus_.
[Illustration:
FIG. 151.—=A=, _Phillipsia gemmulifera_, Phill., Carboniferous. =B=,
_Arethusina konincki_, Barr., Ordovician. =C=, _Dalmanites
limulurus_, Green, Silurian. (After Hall.) =D=, _Cheirurus
insignis_, Beyr., Silurian. =E=, _Deiphon forbesi_, Barr., Silurian.
=F=, _Acidaspis dufrenoyi_, Barr., Silurian. (=A=, =B=, from Zittel;
=D=, =E=, =F=, after Barrande; natural size.)
]
=Fam. 12. Cheiruridae= (Fig. 151, D, E).—The glabella is convex or
inflated, and distinctly defined. The facial suture passes from the
lateral to the front margin. The free cheeks are small, and the eyes
usually rather small. There are from nine to eighteen (usually eleven)
thoracic segments; the pleurae have ridges or grooves and free ends. The
pygidium is small, consisting of from three to five segments often
produced into spines. Upper Cambrian to Devonian. Genera: _Cheirurus_,
_Deiphon_, _Placoparia_, _Sphaerexochus_, _Amphion_, _Staurocephalus_.
=Fam. 13. Proëtidae= (Figs. 150, G; 151, A, B).—The body is rather
small, and the head forms about a third of its entire length. The
glabella is sharply defined, and its furrows are sometimes indistinct;
the posterior furrow curves backward to the neck-furrow, thus limiting a
basal lobe on each side of the glabella. The eyes are often large (Fig.
150, G); but in _Arethusina_ (Fig. 151, B), in which an eye-line is
present, they are small. The facial sutures pass from the posterior to
the anterior margin. The free cheeks are large. There are from eight to
twenty-two thoracic segments with grooved pleurae. The pygidium is
usually formed of numerous segments, and its margin is usually entire.
Ordovician to Permian. Genera: _Proëtus_, _Arethusina_, _Cyphaspis_,
_Phillipsia_, _Griffithides_, _Brachymetopus_, _Dechenella_.[203]
=Fam. 14. Encrinuridae.=—The cephalic shield is ornamented with
tubercles. The free cheeks are narrow, and the eyes very small. The
facial suture extends from the lateral margin (or from the genal angle)
to the anterior margin. There are from ten to twelve thoracic segments
with ridged pleurae. On the axis of the pygidium numerous segments are
seen, but usually fewer are indicated on the lateral parts. Ordovician
and Silurian. Genera: _Encrinurus_, _Cybele_, _Dindymene_.
=Fam. 15. Acidaspidae= (Fig. 151, F).—The cephalic shield is broad, with
a spinose margin, genal spines, and sometimes spines on the neck-ring.
The glabella has a longitudinal furrow on each side, due to the backward
bending of the lateral furrows. The facial suture passes from the
posterior border (near the genal angle) to the anterior border. The free
cheeks are large; the eyes small. There are from eight to ten thoracic
segments with ridged pleurae, which are produced into long backwardly
directed spines. The pygidium is short, and is formed of two or three
segments with long spines at the margin. Ordovician to Devonian. Genus:
_Acidaspis_.
=Fam. 16. Lichadidae.=—The body is broad, with a granular dorsal
surface. The cephalic shield is small and short, with spinose genal
angles. The glabella is broad, and its anterior furrows are directed
backwards, limiting a convex median lobe and some lateral lobes. The
facial suture extends from the posterior to the anterior margin. There
are nine or ten thoracic segments with grooved pleurae, which have
pointed ends. The pygidium is large and triangular, with a short axis
and a toothed margin. Ordovician to Devonian. Genus: _Lichas_
(sub-genera, _Arges_, _Dicranogmus_, _Conolichas_, _Ceratolichas_).
INTRODUCTION TO ARACHNIDA,
AND
XIPHOSURA
BY
A. E. SHIPLEY, M.A., F.R.S.
Fellow of Christ’s College, Cambridge, and Reader in Zoology in the
University