Page:EB1911 - Volume 23.djvu/165

Rh and as such is crossed by the auditory columellar chain. The infra-temporal bridge or jugal arch is formed by the jugal (qj in fig. 12), which joins the descending process of the squamosal, and the quadrato-jugal, which is very small and partly fused with the lateral side of the quadrate. Now, between the quadrate on the one side and the squamoso+quadrato-jugal+jugal on the other, is enclosed a gap, met with only in Sphenodon of recent reptiles. This fourth, or quadrato-squamosal foramen, with its squamoso-quadrato-jugal bridge, is, as a rule, not mentioned, being too small to be obvious. The quadrate is very firmly fixed. On the ventral side of the cranium we notice the broad and long bony palate, the large vomers, and the pterygoids meeting in the middle line; aside of the vomers are the long posterior nares; posteriorly the pterygoids diverge to rest upon short basi-sphenoid processes, and they articulate by short flanges with the quadrates.

The occipital condyle is kidney-shaped, triple, composed of the basi and the lateral occipital. The dorsal median roof of the cranium is formed by the paired parietal, near their anterior symphysis with the large pineal foramen, the paired frontals, nasals and premaxillaries. The outer nares are surrounded by the premaxillaries, maxillaries and nasals. Prefrontals and postfrontals exist. There is a complete cartilaginous, interorbital septum, and a cranial columella, a pair of upright buttresses arising in the alisphenoidal walls, connecting the parietal with the pterygoids. The hyoid apparatus consists of a narrow base, with three pairs of arches; of these the first or hyoid arch is variously connected with the cranium near the paroccipital process, or with the extracolumella (see Middle Ear, below); the others are a long and stout pair of first and a smaller pair of second bronchial arches.

Crocodiles.—The temporal region is still bridged over by three arches, dividing the whole fossa into three, very much as in Sphenodon. The supratemporal fora men is bordered by the parietal, postfrontal (postorbital absent) and squamosal. The posttemporal foramen is very much reduced, sometimes to a narrow passage between the parietal, occipital and squamosal, because the latter bone forms an extensive suture with the paroccipital process. The infratemporal or lateral fossa is wide and rather shallow, bordered above by the post frontal and squamosal, in front by the post frontal and jugal, below by the jugal and quadrato-jugal, behind by the latter, the quadrate, tip of the par occipital and the squamosal. The quadrato-jugal being long and in an almost horizontal position, being wedged in between the jugal and nearly the whole length of the lateral edge of the quadrate, and there being no squamoso-quadrato-jugal bridge, the fourth foramen of Sphenodon is absent. The middle-ear cavity is reduced to a complicated system of narrow passages; one for the passage of the extra-columellar-mandibular string of the auditory chain (see Ear, below), between the quadrate, paroccipital and lateral occipital bones; another passage (Eustachian) opens in the roof of the mouth, between basioccipital and basisphenoid; a third joins that of the other side and forms with it a median opening between the same bones, just behind the posterior pterygoid border of the choanae. These nares, being in the recent crocodiles shifted as far back as possible, communicate with the outer nostrils by very long passages, formed by the whole length of the pterygoids, palatines, maxillaries, vomers and pre-maxillaries, all of which form a long median suture. But this long bony palatal roof is interrupted by a pair of large palatal foramina, bordered usually by palatine, pterygoid, ectopterygoid, or transverse bone and maxillary. On the dorsal side of the Cranium we notice the parietal fused into an unpaired bone, without a pineal hole and the likewise unpaired frontal. There are a pair of postfrontals, prefrontals and lacrymals perforated by the naso-lacrymal duct. The nasals vary much in length, mostly in conformity with that of the maxillaries; as a rule they reach the short premaxillaries, but not always the nasal groove. (For taxonomic detail see under .)

The occipital condyle is formed mainly by the basioccipital, which always borders part of the foramen magnum, but the

lateral occipitals each send a flange to it, which in immature specimens still partakes of the articulation with the atlas. The opisthotic and eniotic bones fuse early with the lateral and with supraoccipital bones; only the proötic remains longer as a separate element, anteriorly with a large hole for the exit of the third branch of the trigeminal nerve. The basisphenoid is scarcely visible, being overlaid by the pterygoids. The presphenoid is larger, continued forwards and upwards into the inter-orbital septum, which remains mostly cartilaginous. Near the anterior and upper margin of the pre-sphenoid is a large notch on either side for the passage of the optic nerve, the three eye muscle nerves and the first branch of the trigeminal. The place of the orbitosphenoids is taken by membrane or cartilaginous continuations of the interorbital septum, but the alisphenoids are large and abut upwards against the frontals and with a lateral flange against the postfrontals. These send down a conspicuous process which forms sutures with an upward process of the jugal and another of the ectopterygoid; it is this compound pillar which partly divides the orbit from the infra temporal or lateral fossa. The size of these and the upper temporal fossae stand in an inverse ratio to each other. The upper fossae are still comparatively large in the long-snouted Gavialis and Tomistoma, whilst these holes almost completely disappear in the alligators, namely, in the broad- and short-snouted members of the order, which chew their prey. In extinct Crocodilians the upper fossae were the larger. The temporo-mandibular muscle which lifts or shuts the lower jaw arises from the walls of the upper fossa, passes beneath the jugal-arch and is inserted upon the supra-angular portion of the lower jaw. In the more recent crocodiles this muscle is more and more superseded by the pterygo-mandibular muscle, which, arising chiefly from the dorsal surface of the much-broadened pterygoid, fills the widened space between the latter and the quadrate, and is inserted into the outer surface of the angular bone. The arrangement of this muscle secures a more advantageous leverage of the jaw, and is capable of more powerful development than the other, which is consequently on the wane—a nice illustration of onward, orthogenetic evolution. The dentary bones of the under jaw form a suture, later a symphysis; this is very long in the long-snouted genera, in which the splenials likewise form a long symphysis; in the others the mandibular symphysis is much shorter and the splenials remain widely separated. The articular bone is short, forms a transverse cup for the quadrate, or a saddle-shaped cup, and is perforated by the Siphomium (see below under Ear). The angle is upturned, formed by the articular, angular and, laterally, by the supra-angular bone; the opercular or counterpart of the splenial lies on the outer side, forming part of the anterior border of the oval foramen in the jaw.

The Chelonian skull agrees in many important features with that of Sphenodon and of the crocodiles, but it is composed of fewer bones, the ectopterygoids, lacrymals and postorbitals being absent, often also the nasals, unless they are fused with the prefrontals. The vomer is unpaired and forms a septum between the nasal passages, which, except in Sphargis, are ventrally roofed over to a variable extent by wings sent out by the palatines, joining the sides of the vomer. Most of the configurations of the other cranial bones are well represented in the accompanying figures. The palatines form a continuous broad floor with the pterygoids, which are extensively and firmly joined to the quad rates and to the basisphenoid. There are no Eustachian tubes. The occipital condyle is distinctly triple and the basioccipital is frequently excluded from the foramen magnum. The lateral occipital early send out a pair of stout wings, the ventral of which joins a stout ventrilateral process of the basioccipital, both forming a thick knob especially in Chelone, and a dorsolateral wing, which broadly joins the large opisthotic bone. This connects the lateral occipital and the supraoccipital with the upper portion of the quadrate. On the top of the quadrate and upon the lateral dorsal portion of this compound transverse process (which of course corresponds to the paroccipital process of crocodiles, &c.) lies the squamosal, about which more presently. The two wings of the lateral