Lacertilia. The zygomatic arch of the Mammalia is formed (cf. also Agamidae) out of the supratemporal arch of Sphenodon, after the loss of the postorbital element and of the quadrato-jugal, the squamosal gaining connexion with the upper, not posterior and ventral, branch of the jugal or malar bone.
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Fig. 17.—Ventral Aspect of Skull of Chelys matamata. bo, basioccipital; bs, basisphenoid; mdl, mandible; oh, opisthotic; pl, palatine; pm, premaxilla; po, proötic; pb, pterygoid; q, quadrate; s, squamosal; v, vomer. |
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Fig. 18.—Lateral Aspect of Skull of Chelys matamata. an, angular; ar, articular; bo, basioccipital; d, dentary; op, opisthotic; m, maxilla; pa, parietal; pm, premaxilla; pr, prefrontal; ps, postfrontal; pt, pterygoid; q, quadrate; s, squamosal; sg, supra-angular. |
The mandibular halves form a complete osseous symphysis, the only instance in reptiles; all the other elements retain their sutures. The articular portion of the articular bone forms several shallow cups and a slight anterior knob, best developed in Chelone. The angular bone does not help to form the posterior upper angle. The coronoid, or complementary element, is often small; the supra-angular and the splenial or opercular are always present, mostly also a pre-splenial wanting in Testudinidae (cf. G. Baur).
The hyoid apparatus is well developed, and sometimes assumes large dimensions, especially in Chelys. The two pairs of “horns” are the first and second branchial arches, whilst the hyoid arches are reduced to a pair of small, frequently only cartilaginous nodules, attached near the anterior corners of the basis linguae, which generally fuses with the os entoglossum in the tip of the tongue. In Chelydidae the long median basal or copular piece forms a semi-canal for the reception of the trachea.
In the skull of the Lacertilia the arcades over the temporal region vary much in composition and numbers. There are at most two arcades and two windows. First the posttemporal arcade, enclosing the posttemporal fenestra, which is framed mainly by the large paroccipital process below and the long parietal process above, both meeting distally, and the quadrate is carried by the paroccipital process. In the corner, in front, where the three bones meet, lies the squamosal, connecting parietal and quadrate. This squamosal, when not too much reduced, has an upper parietal and an anterior horizontal arm; the latter is essential for the formation of the second horizontal arcade, which makes the lower border of the supra-temporal window. The infra-temporal arcade, namely a quadrato-jugal+jugal arch, is absent in all Lacertilians owing to the complete absence of the quadrato-jugal element.
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Fig. 19.—Skull of Chlamydosaurus kingii (old male), showing much differentiated teeth. 1, ventral aspect; 2, posterior; 3, profile, showing the enormous process at the hinder end of the lower jaw. |
In Heloderma and Geckos the post temporal is the only arcade.
In the Amphisbaenids and in Aniella, practically also in
Anelytropsis, all the arcades are lost. All the other families
of lizards and the chameleons have two arcades. We begin the
description of the horizontal arcade with those families in which
it is most complete, and most like that of Sphenodon. In
Varanus it is formed by four bones. The postfrontal is short;
to it is attached the post orbital, which sends a long horizontal
process to join the squamosal[1] splint, and this connects with the upper anterior end of the quadrate; between the quadrate,
the squamosal and the long parietal process lies the likewise
splint-like supratemporal, attached by most of its length to the
parietal process. The jugal has only one arm, and this connects
the maxilla with the postorbital, completing the posterior
orbital border. There is a wide gap between jugal and quadrate.
In Tejidae the arcade is the same, but the squamosal reaches the
jugal, both meeting the post orbital. In Lacerta the arcade is
essentially the same, but the window is completely filled up by
the post frontal, which extends so far back as to reach the
Fig. 20.—Dorsal aspect of
skull of Heloderma horridum.
f, frontal; j, jugal; l, lachrymal;
m, maxilla; n, nasal;
pa, parietal, pm, premaxilla;
pr, prefrontal; ps,
postfrontal; pt, pterygoid; q,
quadrate; s, squamosal; so,
supraoccipital.
supratemporal. In the Agamidae the
arcade is strong and simplified.
Postfrontal and postorbital are
represented by one forked piece.
This squamosal and the
postfrontal mass are connected by the
upper, much up-curved end of the
jugal, which is thrust between
them. This arrangement is
further emphasized in Iguana, the
upper end of the jugal being much
enlarged so as to form the greater
portion of the arcade, and keeping
the post frontal mass and the
simple squamosal widely asunder.
In Heloderma post- and prefrontals
are in contact with each other,
separating the frontal bone from
the orbit; the jugal joins only
the prefrontal, and there is no
further arcade whatever. A
vestige of a supratemporal (?)
lies on the outside of the base
of the squamosal, between s and
q in fig. 20.
The chameleons are peculiar. The posttemporal arcade,
spanning a wide space, is formed by a long process of the
Fig. 21.—Skull of Chamaeleon vulgaris.
ag, angular; ar, articular; bs, basisphenoid;
d, dentary; j, jugal; m, maxilla;
me, median ethmoid; p¹ and p², parietals;
pl, palatine; pr, prefrontal; pt,
pterygoid; q, quadrate; sg, supra-angular;
so, supraoccipital; sq, squamosal.
supratemporal-squamosal,
which is directed up-
and backwards to join
the parietal, which
extends back by a long
unpaired process. The
horizontal arch is broad
and short, squamosal
and postfrontal, forming
a broad suture;
below they are joined
by the jugal; above
the suture lies, in
chameleon, a tiny piece,
perhaps a vestige of
the dislodged
postorbital.
The jugal bones, to continue the description, of the appendicular parts of the skull, are firmly joined to lateral processes of the pterygoids by the ectopterygoids; further forwards they are extensively connected with the maxillaries. These rest against strong transverse palatine processes. The palatines form a medium symphysis; posteriorly they diverge together with the pterygoids, which articulate with the quadrates and with the basisphenoid by a pair of strong basipterygoid processes. A slender vertical rod of bone, the columella cranii, arises from the dorsal surface of each pterygoid and, passing at a distance from the cranial capsule, is sutured to a short lateroventral process of the parietals. Such a pair of columellæ exists in nearly all Lacertilia (distinguished by many systematists as Kionocrania) with the exception of the chameleons and the Amphisbaenidae. In many lizards, however, this columella, or epipterygoid, does not quite reach the parietal, leaning instead against the proötic; possibly it has been evolved out of the alisphenoid, and Chelonians seem to support this view. The premaxillary bone is single, except in the Skinks and in some Geckos; ventrally it touches the vomers which vary much in size; they are always paired although suturally connected; posteriorly they pass into, and fuse with, the palatines before these send off their maxillary processes. Between the vomer and its maxillary is a longitudinal hole. Often, e.g. in Lacerta, the vomers enclose a median hole near their anterior end, for Jacobson’s organ. Dorsally the premaxilla sends a median process backwards to the nasals. These are paired, and fuse together only in Uroplates and in Varanus. The external nasal fossae are sometimes very large, and their anterior half appears blocked by the ossified turbinals, e.g. in Varanus and Tejus. Prefrontals are always present, often fused with the lacrymals; in Heloderma, in Aniella and in chameleons the prefrontals extend so far back as to meet the postfrontals, excluding thereby the frontals from the orbital rim. The frontals are either paired, as in Varanus, Lacertidae, Heloderma, Anguidae, Scincidae, Anelytropsidae, Aniella, Amphisbaenidae, and in some Geckoninae; or they are fused into one bone, as in the Eublepharinae, chameleons, Tejidae, Iguanidae, Agamidae, Xenosaurus. The parietal are double in the Geckos, in Uroplates and Xantusia; in all the others they form one coössified mass, generally with a pineal foramen, except in Eublepharinae, Amphisbaenidae, Tejidae, in Aniella and other degraded forms. In the majority the pineal foramen lies in the middle of the parietal, but in the Iguanidae it is near the frontal, and actually in the frontal in chameleons.
As regards the brain-case, there is a cartilaginous interorbital septum, connected posteriorly with the slender, bony presphenoid; ventrally on to this is fused a vestige of the parasphenoid, a narrow and thin splint which sometimes can be dislodged. The whole of the anterior wall of the brain-case is membranous, excepting a pair of separate ossifications, which do but rarely touch any of the cranial bones, as frontal, parietal or proötics. The ossifications are irregular in shape, each sending out a downward process which curves inwards almost to meet its fellow; between these issue the olfactory lobes. W. K. Parker recognized them as the alisphenoids; E. D. Cope named them postoptics, and remarked that in Sphenodon they coexist with an orbitosphenoid bone. The proötic has a notch in its anterior lateral margin for the passage of the trigeminal nerve. The opisthotic portion of the petrosal mass is intimately fused with the lateral occipital bones and their paroccipital process, and sometimes, e.g. Tejus, encloses with them many intricate recesses of the middle ear-chamber, which extend also into hollow and swollen thick downward processes of the basioccipital. These cavities of both sides communicate with each other through the cancellous substance of the basioccipital and basisphenoid. There are no Eustachian tubes opening into the mouth through the base of the skull.
The occipital condyle is tripartite, the lateral occipital partaking of the articulation; very rarely, e.g. in Amphisbaenidae (see fig. 22), the basioccipital portion is so much reduced that the skull articulates by two very broad condyles.
The halves of the under jaw are but loosely united, either by ligament only or by an at least very movable suture. The jaw is compound and the numerous constituent bones mostly retain their sutures. Besides the dentary and articular, angular and supra-angular on the lateral side, and the opercular or splenial on the inner side, there lies on the dorsal side the coronoid, six pairs in all. The posterior angle of the jaw
- ↑ There is a much-debated question of the homologies of the one or two elements, both apparently membrane bones, which connect the upper end of the quadrate with the parietal and with the supratemporal arch. The question becomes acute in the snakes, whether the single element connecting skull and quadrate has to be called squamosal or supratemporal. Space forbids here to expound the matter, which has been very ably reviewed by S. W. Williston (“Temporal Arches in the Reptilia,” Biolog. Bulletin, vii. No. 4, 1904, pp. 175-192; cf. also F. W. Thyng, Tufts College Studies, II. 2, 1906). About ten different names have been applied to these two elements, and two, namely, squamosal and supratemporal, are being used quite promiscuously. When only one element is present, the present writer uses the term squamosal, and there are reasons making it probable that this element is the squamosum of mammals. When both elements are present, the more ventral or lateral of the two is termed squamosal, that which always helps to form the supratemporal bridge, generally with the post orbital, sometimes also with the jugal. The more dorsal element is mentioned as supratemporal; it is always smaller, and mostly restricted to the corner between the squamosal and the parietal process against which it rests. Either of these two elements articulate with the quadrate. Both elements are Present in Labyrinthodonts and in most of the extinct groups o reptiles; among recent forms in Lacertidae, Varanidae, Tejidae; one three-armed piece in Sphenadon, chameleons and crocodiles, without, in Sphenodon at least, any trace of a compound nature; one piece. forked, in Agamidae; one simple piece in most of the other Lacertilia, and in snakes.