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the suspensorium of the lower jaw : it is articulated by an
elongated surface with the otic cartilage, and is applied in
front to the hinder edge of the upper mandibular seg-
ment. The remainder of the hyoid arch, and also the
branchial arches, become more or less segmented, but not
remarkably modified. Median basal pieces connect the
bars of opposite sides.

206. The proportion of the chondrocranium which
is replaced by osseous deposit is but small. The occi-
pital ring has four bones, but they are separated by car-
tilage. The feeble basi- and alisphenoids, with a peculiar
massive orbitospheiioid produced by the junction of a pair
of bones, are the other chief cranial ossifications in car-
tilage, besides those related to the ear-capsules. Each of
these acquires five bony centres, of which the pterotic is
important as overhanging the hyomandibular articulation,
while the prootic encroaches largely on the proper cranial
floor. The intrinsic ossifications of the facial bars are very
distinct, their history is simple, and recapitulation is un-
necessary.

207. The subdermal or membrane bones are very nu-
merous, the principal appearing sooner than the cartUage-
bones. Very important elements occur in relation to the



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III.} THE SKULLS OF FISHES. 83

base and roof of the cranium ; they are azygous beneath,
and mostly paired above. The extent and relations of the
paraspbenoid demand especial attention. The precranial,
palatine, and mandibular cartilages have series of bones
related to them as splints; while the large orbits are
partly protected by the outward extension of bones related
to the cranial roof, and partly by special circumorbital
bones. The great opercular and branchiostegal membranes
have their own osseous skeleton, while the branchial arches
possess many small tooth-like parostoses.



APPENDIX ON THE SKULLS OF FISHES.

208. The skulls of most osseouH fifthes are oonstmcted tab-
staotiuUy like that of the Salmon : cartilage may persist to a greater
or less extent, and aiichyloBis of bones may take place. In Siluroid
and Cyprinoid fishes, however, an interorbital septum is not formed.
The substance of the ethmoidal cartilage may become partiaUy
oisified by median (Cod) or paired (Pike) bones. In the Muramoids
the trabeculse become merely narrow bands 6f cartilage in the
orbital region, arched forwards and upwards upon the paraspbenoid.
Anteriorly there is a median vertically-crested ethmoidal bone,
deeply grooved beneath to receive a high crest from the dentigerous
vomer. On either side of this crest lies a distinct simple trabecular
rod. The olfactory nerves as they pass to the nasal sacs are en-
closed by a pair of thick separate ectoethmoidal cartilages; abova
the membranous olfactory capsule is a small nasal bone. Hiere are
no premaxillaries ; the vomer, dentigerous all the way, runs to the an-
terior extremity of the beak ; the maxillaries are large and dentigerous.
^e hyomandibular is very large, with two distinct heads widely
separated: its distal part is directed downwards and forwards;
the symplectic is distinct. Between these two bones the rest of
the hyoid arch is attached by the interhyal ligament. Hie epi- and
ceratohyals are about equal in size, and there is no bypohyaL The
apex of the suspensoriaJ part of the mandibular arch is unossified,
and is let down to a position opposite the interhyal ligament,
being embraced by splint like processes of bone from the hyoman-
dibular. There is a rod-like quadrate, and in front uf it a minute tri-

6—2



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84 MORPHOLOGY OF THE SKULL. [CHAP.

angulsir process of cartilage — all the pterygoid cartilage that exists :
above it is a styloid palato-pterygoid bone. The lower jaw possesses,
besides dentary and articular, a small thick coronoid on the inner
face, wedged between the two larger bones.

209. The SUuroids resemble in important respects the holos-
tean Ganoids ; many of them exhibit a remarkable combination of
ganoid dermal plates with strongly ossified cranial cartilage. They
agree with certain Ganoids, and differ from other Teleosteans in
having no subopercular. In the Siluroid Clarias capensis the two
dentigerous premaxillaries occupy a large part of the margin of the
fore face, and behind them is a crescentic tooth-t^earing plate, the
anterior part of the single vomer. The maxillaries are minute, and
bear a long cirriform filament ; above them each nasal capsule has an
ossification comparable to a septo-maxillary (chap. v.). The nasal sacs
are very small ; so are the orbits, which are placed very far forwards.
The anterior part of the face is covered by a large broad dermo-eth-
moid. Even in the adult, small unclosed spaces persist between the
frontals and in the supraoccipital region. The palatine is a distinct
rod-like bone attached to the lateral ethmoidal region, carrying the
maxillary, and lying partly above and within the pterygoid and the
mesopterygoid. Tlie quadrate comes far forwards, and there is no
itetapttrygoid. By comparing this skull with a Salamandrine form
many instructive points of agreement are discovered.

The Skull in Ganoids,

210. The holostean Ganoids have their chondrocranium ossified
essentially like the Teleosteans, certain bones being sometimes

• missing. The single jugular plate of bone developed between the
mandibular rami of Amia, and the two plates found in Polypterus in
the same region, are structures not plainly comparable with anything
in the Salmon. In other respects the student will find little diffi-
culty in interpreting these skulls.

In Polypterus\ for example, large basal and superior fontanelles
persist in the chondrocranium, which is ossified by an occipital ring of
bones, by otic masses representing opisthotic and epiotic bones con-
joined, by sphenoidal walls united in the cranial floor behind the

1 See Traquair, " On the Cranial Osteology of Polypterus," /otim.
AnaU vol. x. p. 166.



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Ill] THE SKULLS OF GANOID FISHES. 85

pituitary body, and also sendiDg bony plates inwards in the anterior
part of the cranial floor, which meet but do not anchylose ; by sphenotics
or " postfrontals," lateral ethmoids (** prefrontals ") ; and a median eth-
moid in the extreme anterior part of the cranial cartilage, sending
back an upper spur which is parosteal. There is no interorbital
septnm. The basioccipital ossiOcation includes at least one vertebral
centrum, the first neural arch being articulated with it above. The
posterior margin of the sphenoid is slightly notched by the foramina
for the hinder divisions of the trigeminal nerve ; near the middle of
its lower margin is the optic foramen, above and behind which the
motor nerves of the eyeball and the first branch of the trigeminal
emerge. The principal roofing bones of the skull are paired parietiils,
frontals, and nasals, which become more or less anchylosed in old
specimens. The septo-maxillaries and median ethmoid appear slightly
on the surface. The parasphenoid extends underneath almost the
whole occipital region, and reaches forwards nearly to the extremity
of the palate : it has remarkable basitemporal processes, and a
minutely denticulated anterior region. The premaxillarles and
maxillaries have considerable palatal plates. The palato-quadrate
cartilages are ossified by small metapterygoids and quadrates, large
pterygoids and mesopterygoids, and very small palatines ; the vomers
continue the line of the pterygoids, within and parallel to the palatal
plates of the premaxillaries and maxillaries. The hyomandibular is
rod-like, bent almost at a right angle in the region of the opercular
condyle, and it has a small additional ossification at its upper end.
There are large preoperculars or squamosals, considenible operculars,
and small suboperculars. The superficial interspaces between the
bones named are filled by many small dermal and membrane bones,
which are more or less modified representatives of the lateral lino
series of scales.

In Amia^ there is a chondrocranium perfectly complete, covered
by a shield of suturally united dermal bones. The occipital
cartilage bunes are all present, two neural arches being articulated
with the hinder part of the basioccipital. The basisphenoid is
represented by a pair of ossifications on either side of the pituitary
fossa; there are large alispheuoids, having a descending plate: the
orhitosphi noid is simple. The ear-capsule has a complete set of
bones, with the exception of the pterotic. The parasphenoid has
^ See Mr Bridge's valuable account in Jour. Anat.^ Vol. xi.



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8G . MORPHOLOGY OF THE SKULL. [CHAP.

basipterygoid wings exteDding to and propping up the sphenotic.
The supracranial dernial shield consists of a median dermo-occipital,
flanked by a pair of sapra-epiotics; of large paired frontals; a pair
of nasals; and an anterior supraethmoid which partly underlies the
nasals. The circumorbital bones are highly developed; there are
two large postorbitals which extend backwards by the side of the
Mipra-epiotics. The palatine series of bones is normal Each
premaxillary has two ascending processes surrounding the nostril
and uniting above it, and beneath the nasal bone. The maxillary
has a jugal behind it. The suspensorium and the branchial arches
need not be remarked upon ; but the lower jaw is singularly inte-
resting. The meckelian cartilage is largely persistent, ossified
proximally by four very distinct centres, three of which enter into
the condylar surface, and distally by a mento-meckelian. The
splenial is in five distinct pieces, one being larger, but all den-
tigerous. The other bones are dentary, angular, and surangular.
The opercular set of bones is complete, with the addition of a
supratemporal.

211. In the Sturgeon there is a very solid cartilaginous skull,
overlaid by dermal bones more or less representing the superficial
bones in Teleosteans, and having a large parasphenoid at its base.
The precranial region is of great extent, forming a massive beak,
surrounded by very generalised dermal bones. The mouth is swung
from a large hyomandibular, which has a small sheath of bone near
the top, and swells out below into a broad solid plate of cartilage.
To this is loosely jointed a symplectic piece, largely ossified, and
turned forwards, bearing the upper and lower jaws. The broad meta-
pterygoid passes towards the middle line in the roof of the mouth,
above and behind the continuous palato-quadrate cartilage which is
partially ossified as pteryg^oid and palatine: sometimes a small quad-
late bone is found near the condyle. The margin of this palatine
tract is flanked by the edentulous maxillary, which reaches to the
quadrate articulation, and bears a styloid jugal. In old specimens there
is a separate mesopterygoid. In the lower jaw the meckelian carti-
lage persists, surrounded by the dentary, and sometimes ossified
anteriorly by a niento-meckelian : there is also a small angular. The
lower division of the hyoid arch starts from the hinder end of the
symplectic, and has interhyal, ceratc^yal, and hypohyal segments.



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III.] THE SKULL OF CEBATODUS. 87

TJie Skull of Ceratodui.

212. In Ceratodus\ the massive cartilaginous cranium is pro-
duced laterally aud inferiorly into a large palato-suspeusorial process,
continuous behind with the ear-capsule. From the latter there pro-
jects backwards a thin wide cartilaginous plate roofing the branchial
chambers. The persistent meckelian cartilage is articulated with the
condyle on the suspensorium at its farthest outward extension. A
small cartilage identified by Prof. Huxley as hyomandibular is found
on the hinder side of the suspensorium, firmly bound to it at its
junction with the cranium proper ; and immediately in front of its
anterior edge the posterior division of the facial nerve emerges from
the cranium. It has a symplectic process embedded in the strong
hyosuspensorial ligament, by which the main hyoid bar is fastened to
the suspensorium. There are cartilaginous representatives of bran-
chiostegal rays. Each of the anterior four branchial arches consists
of a long ventral and a short dorsal piece of cartilage; there are
rudiments of basibranchials. The fifth arch consists of a single piece
of cartilage on each side, curved fonv ar Js and united with the princi-
pal piece of the fourth arch, both above and below. Two pairs of
small labial cartilages have been found defending the posterior narial
orifices.

213. The cartilaginous cranium is covered by an osseous shield
consisting of two median bones, one behind the other, occupying almost
the entire length of the skull, and of two pairs of lateral bones, one
outside the other, extending over the hinder two-thirds of the head,
and lying principally above the otic cartilage. The hinder and
larger part of this osseous shield lies upon the large temporal
muscles which entirely cover the chondrocranium. The outer bone
on each side appears to be homologous with the preopercular of
Teleosteans and the squamosal of higher vertebrates: it has an
elongated process running down nearly to the quadrate articulation.
A fibrous band stretching between the antorbital process and the
ventral end of the suspensorium contains three suborbital bones.
The crania)^ floor is supported by a large parasphenoid which
ext^ds backwards to a point just beyond the attachment of
the third pair of ribs, several anterior vertebrae being, as in the

^ Prof. Huxley's description is here principally followed (Proc, Zooln>
Soc. 1876) ; see also Giinther, Phil, Trans, 1871.



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88 MORPHOLOGY OF THE SKULL. [CHAP.

Sturgeon and other Ganoids, confluent with the skull, so that the
hinder boundary of the latter has to be imagined between the exits
of the pneumogastric and of the first spinal nerve. Just in front of
this boundary there lies on either side, deep in the substance of
the cartilage, a hollow cone of bone (exoccipital). The parasphenoid
is narrow behind and lozenge-shaped anteriorly, with two pointed
basitemporal wings at the junction of the two regions. In front of
these processes it is conterminous with the two elongated palate-
pterygoid bones, which extend inwards from the region of the condyle
for the mandible, to meet each other in the middle line for some
distance in front of the parasphenoid. Tlie fore part of each palato-
pterygoid has anchylosed to it a large dentary plate lying obliquely,
and bearing prominent ridges on its external region. Vomers are
represented merely by the bases of the pair of anterior trenchant
teeth planted on the ethmoidal cartilage, meeting each other in the
middle line and diverging outwards and backwards at a right angle
to one another. A downward process from the inner and anterior
of the lateral bones of the cranial roof meets the fore part of the
palatopterygoid. There is no interorbital septum. The meckelian
cartilage is sheathed by dentary, splenial, and angular elements, the
spleuial bearing a dentary plate corresponding to the large palatal
one above described. In addition to the preopercular or squamosal
already mentioned, there is a principal opercular and a smaller rod-
like interopercular. The only other ossification to be noted is the
ensheathing bone oi the large hyoid bar.

214. The skull of Lepidosiren presents many features of resem-
blance to that of Ceratodus, especially in its cartilaginous parts ; no
tract, however, identifiable with the hyomandibular appears to have
been clearly made out. There are two exoccipitals, a large para-
sphenoid extending partially beneath the vertebral column, a single
roofing bone from the occipital to the ethmoidal region (called
parieto-frontal by Prof. Huxley), a pair of nasals, or one median
supraethmoid, and a great '* supraorbital" on each side, which
passes back to the extremity of the head at a considerable height
above the other cranial structures. The "parieto-frontal" and
parasphenoid send processes towards one another in the side walls
of the cranium, which unite in front of the exit of the hinder division
of the trigeminal nerve. The suspeusorium is continuous with the
de wall of the cranium, and has on its anterior and outer margin



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III.] THE SKULL OF CHIMiERA. 89

the large palatopterygoid, meeting its fellow in the middle line, and
bearing deutary plates. The vomerine rei^on bears teeth but no
distinct bone at their base. Prof. Huxley says^ **The parasphenoid,
the rudimentary vomers, and the pterygopalatine plates correspond
in the two genera (Lepidosiren and Ceratodus). The exoccipitals
are much larger in Lepidosiren. The descending process or pre-
opercular part of the squamosal is best developed in Lepidosiren,
whilst its dorsal part (proper squamosal) is larger in Ceratodus. In
both, there are two opercular bones, an operculum and an interoper-
culum ; in Lepidosiren, as in Ceratodus, there are cartilaginous plates
attached to the inner faces of these bones. The branchial apparatus
of Lepidosiren differs from that of Ceratodus mainly in the greater
number of complete branchial arches.^' It must be confessed that
a knowledge of the development of the skull of Lepidosiren is very
much needed before a satisfactory account of it can be giveiu

215. In Chimcera^ the suspensonum is continuous with the side
of the brain-case, but its principal development is :ar foi wards, carry-
ing the condyle for the mandible. There are two palatine and two
mandibular teeth fittiug into one another, and two vomerine teeth
exist in front of these, as in Ceratodus and Lepidosiren. The
cranial cartilage is produced into a remarkable interurbit-il crest
cibove the anterior part of the brain. The notochord has disap-
peared, and the skull is articulated with the anterior coalesced
vertebrse. The hyoid arch is substantially similar to the branchials,
though larger. It terminates dorsally in a flat expanded triangular
piece, connected with the superjacent floor of the skull by muscles
aud ligaments, but by no direct articulation. The dorsal pieces of
the succeeding branchial arches have the same form and attachuients,
and unite with the ventral segments at a sharp angle. An opercular
membrane covers the gill- clefts. There are large labial cartilages.

216. In Sharks and Rays there is a great diversity of form and
relative size of parts, but the only members of the group that need
special notice are Notidanus (Heptanchus and Uexanchus) aud
Cestracion. In Notidanus tl:e hyomaudibular is much reduced, and
only partially supports the jaws ; the gape is of great extent, and
the articulation of the lower jaw is far back ; the upper jaw is much

1 L c. p. 38.

2 Huxley, /. c. p. 40.



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90 MORPHOLOGY OF THE SKULL. [CHAP. III.

enlarged vertically behind its middle, and forms a definite articula-
tion with the strong postorbital or sphenotic process of the cranium.
Fiurther forwards there is also a small procens which is tied to the
basal prominence of the skull behind and below the optic foramen.
There is no spiracular cartilaga Heptanclius has seven brancliial
arches, Hexanchus six. In Cestracion the fore part of the skull is
very long in proportion to the posterior; the hyoniandibular is small,
and articulated with the cranium low down and far back^ in a com-
pletely normal relation to the exit of the facial nerve ; whUe the
upper jaw has an almost semicircular superior edge, and derives its
chief support from an elongated area at tlie base of the craiuuiB m
front of the orbit. There is a distinct spiracular cartBage. In Cen-
trophorus there are three carl^agiooiis rajs dose together in the
anterior waU of the spiracla

217. The descriptions here given are considered not to go far-
ther in identification than the facts of development at present
known will warrant. An independent judgment can be formed upon
iliem by each observer. The skuils of the Lampreys and Myxinoids
are not similarly treated, owing to our ignorance of their develop-
ment, and of the extent to which our present knowledge may be
sufficient for their comprehension ^ Generally speaking, it may be
stated that they have a simple chondrocranium arising in the fore
part in the form of trabeculse and in some retaining that form per-
manently, together with more or less rudimentary visceral arches.
To these elements very large labial developments are added. It
does not appear that sufficiently sure grounds exist for definite con-
clusions respecting the relation of the structures found in the head
f>f Amphioxus to the skulls of other vertebrates, although highly
interesting and instructive speculations have been made on the
bubject*-*.

1 See Huxley, " On Petromyzon," Jmtr, Anat. x. 412.
' See Huxley, ** On Amphioxus," Proc, Roy, Soc, 1875.



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CHAPTER IV.

THE SKUIJi OF THE AXOLOTL.

First Stage: Embryo Axolotls, a day or two befors
hatching.

218. The embryo lies coiled crescentically in its gela-
tinous envelope, and is only three lines in length. The
tail is very short; the abdomen is protuberant, enclos-
ing the yelk; the head constitutes fully one- third of the
length of the little creature. The head is broadest over its
hinder part, in the gill-region, and tapers gently forwards.
The mesocephalic flexure is fully established, and the brain-
vesicles are strongly protuberant. The third vesicle is well
seen above, occupying much the greater portion of the
upper surface of the head. In shape it is elongated oval,
pointed behind, and covered by a thin dermis, beneath
which there is a very watery stroma down to nearly half
the depth of the chamber. The second vesicle is sub-
globular, and forms the anterior teiinination of the head.
The first vesicle is small and completely on the under sur-
face, below the hinder half of the second vesicle.

219. The sense-capsules lie in a lateral oblique line,
from just behind the middle of the third vesicle to the
side of the first. The olfactory pit consists of an annular
elevation surrounding a crater-like depression, lying on the
supero-posterior face of the first vesicle; it is the smallest
of the sense- capsules. Immediately above and behind it
is the commencing eyeball, a reniform mass of cells with a
stalk looking downwards and backwards. A space inter-



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92 MORPHOLOGY OF THE SKULL. [CHAP.

venes between this and the next organ, the auditory, which
is also at a higher level: it is a reniform mass with its
stalk above.

220. Looking on the under surface of the head, there
is seen a narrow crescentic membrane convex backwards,
behind the first vesicle. This is the nasofrontal process,
forming a curved commissure between the two olfactory
sacs. Behind this there is a slight depression, which is in
front of the convex anterior edge of the mandibular fold :
the boundaries of the depression being convex towards
each other, it is necessarily broad at each side and narrow
in the middle, so as to be somewhat hourglass-shaped.
This is all that is manifest externally as a rudiment of the
mouth.

221. Behind the mouth two rib-like elevations lie
transversely one behind the other, ascending laterally. The
anterior of these reaches nearly to the eyeball, the pos-
terior to the auditory rudiment. There is no definite
solution of continuity between these two bars, which are the
first indications of the mandibular and hyoid arches. They
are flat below and project somewhat forwards in the mid
line. On either side of the rudimentary mouth-cavity below
the eyeball, and immediately behind the nasal rim, is a
small bud of tissue (maxillopalatine) which is an outgrowth
of the upper part of the mandibular arch.

222. The hinder margin of the hyoid arch is crescen-
tic in the middle ventral line, the concavity being back-
wardly directed. The horns of the crescent project back-
wards on each side, distinct from the body-wall and over-
lapping the structures behind. These are the rudiments
of the opercular folds. The under surface of the body im-
mediately behind the hyoid arch presents no other struc-
tures besides the continuous thin skin, through which the
heart is just visible. Infero-laterally, behind each opercu-
lar fold, three oblique slits are seen ascending backwards,
leading into the pharynx or fore part of the alimentary
cavity. Thus there is marked out a series of visceral



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IV.] THE AXOLOTT.: FIRST STAGE. 93

arches behind the hyoid, and the fissures must be called
the second, third, and fourth visceral clefts.

223. The visceral arches are slightly distinguishable



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