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Show 262 ON THE S'l'RUC'l'URE OF 'l'HE SKULL. rrhe premaxillre enter largely into the composition of both the upper and under regions of the snout. As has been already stated, they unite in front of the nasal bones, so as to exclude the latter from the anterior uares, as is the case in some Grocodilia. The maxillary bones send horizontally inwards a broad and long palatine process. This, Jike the corresponding process of the palatine bone, is separated from its fellow in the middle line, for some distance, by the vomer. On tho left side of the specimen from which this description is taken there is a distinct large triangular lachrymal (Fig. 101); it is imperforate, and situated altogether upon the side of the face. An oblique suture extends downwards and forwards fron1 that which separates this lachrymal, inferiorly, from the adjacent bones, and seems to 1nark off the jugal from the maxillary bone. On the right side neither this suture exists, nor any indication of a distinct lachrymal. The essential characters of the Probo ... cidean cranium are best displayed in the frotal Elephant, a· the sutures become obliterated, and the true form of the skull is disguised by the enormous development of the air-chambers between the tables of the skull, in the adult, Fig. 103 represents the longitudinally and vertically bisected skull of such an Elephant. The whole basi-cranial axis is slightly concave upwards. The ba i-occipital and the basisphenoid, the presphenoid, and the ethmoid are already so completely anchylosed that the traces of their primitive distinctness have almost disappeared. On the other hand, the presphenoid and the basi-sphenoid are widely separated by the remains of a synchondrosis. The occipital angle i a out 90°, the olfactory angle 160° to 170°. The frontals enter as much into the front wall as into the roof of the skull, and extend largely down upon its sides. Anteriorly and externally they are prolonged into great arched supra-orbital processes, which form the roofs of the orbits. The parietals are narrower in the middle line of the vertex than anywhere else, being encroached upon by the frontals, anteriorly, and by the supra-occipital behind. Infero-laterally, the I arietals widen out very Intwh and extend far down into the THE SKULLS 01!., MAMMALIA. 263 temporal fossre, where they unite, in front, with the apiees of the tolerably large orbito-sphenoids, and behind, with the periotic and supra-occipital. Below the inferior margin of the parietals the squamosals appear largely in the lateral wall of the skull. The alisphenojds are very small, and are directed horizontally outwards. The foramen for the exit of the third division of the trigeminal is between the hinder margin of the bone and the periotic. The latter bone has a considerable proportional size, and is devoid of any cerebellar fossa. Fig. 100. .Pa lm!llllllfiloillll";!retr.IJs Fig. 103.-Longitudinally and vertically bisected cranium of a freta! Elephant (EtepAas Indt'cus). On the exterior of the skull the squamosal joins the ex-occipital, so that no "pars mastoidea " appears upon the surface. The post-glenoidal and post-auditory processes of the squamosal are very large, and bend towards one another inferiorly, so as to meet (in the adult skull) and form a spurious external auditory meatus. But besides this, there is a true external auditory meatus which is, as usual, an outgrowth frmn the tympanic. The latter bone is very large and bullate. It is grooved anteriorly by the carotid, and the short styloid process appears between it, the squamosal, and the ex-occipital. The tympanic and the periotic are anchylosed together and wedged into the space left between the ex-occipital, squamosal, parietal, alisphenoid, and the basi-cranial axis. The very short nasal bones (absent in the specimen figured) |
