440 ANATOMY OF VERTEBRATES. 



The portions of the olfactory capsule closing the anterior orifice 

 of the cranial cavity form extensive ' cribriform ' plates. The 

 frontal, fig. 295, ii, is excessively expanded by the air-cells, fig. 

 296 ; its hind border is convex, its front one concave, and ex- 

 tended outward to form the superorbital ridge. The nasals, 15, 

 are short, triangular, and pneumatic : they ultimately coalesce 

 with the frontals. The mastoid is confluent with the squamosal, 

 and, bending forward to near the back part of the zygomatic pro- 

 cess, circumscribes the meatus auditorius externus. The tympanic 

 completes the inner part of the meatus, contributes to the back 

 part of the glenoid cavity, and expands into a broad horizontal 

 plate supporting the large ear-drum : it early coalesces with the 

 petrosal. The apex of this bone is grooved by the ento-carotid. 



The epencephalic compartment of the cerebral cavity, fig. 

 296, e, as in Lissencephala, is wholly behind the pros- and mes- 

 encephalic ones : the rhinencephalic compartment is well defined. 

 The ' sella ' has slightly-marked clinoid processes. The orbits 

 are continuous with the large temporal fossae. The palatines 

 form the posterior half of the intermolar part of the roof of the 

 mouth, and bound the hinder nostril ; they soon coalesce with the 

 pterygoids and maxillaries, 21 : these are remarkable for the large 

 proportional size of their alveolar part, in advance of which the 

 bone extends upward to be wedged between the frontal and pre- 

 maxillary, downward and forward to strengthen the socket of the 

 tusk, and backward to form the anterior pier of the zygomatic 

 arch and the lower part of the orbit. The maxillary is perforated 

 by a large antorbital foramen. The premaxillary, figs. 152, 

 295, 22, mainly consists of the part which lodges the base of the 

 great tusk : but its ascending portion reaches the frontal, 11, and 

 excludes, as in Rodents, the maxillary from the nasal : the alveolar 

 part is grooved mesially by the long incisive canal. Both maxil- 

 lary and premaxillary are pneumatic, fig» 296. The mandible, 

 fig. 295, z, is short, the ascending being as extensive as the 

 horizontal ramus, and being also excavated for the formative 

 alveolus of the succeeding molar. The condyle is small, convex, 

 rising above the coronoid process, which is low and projects 

 obliquely forward. The dental canal is wide in reference to the 

 unceasing supply of material for the growth of the great molars. 

 The symphysis is short, small, pointed : in some extinct Probos- 

 cidians it was excavated for the alveoli of a pair of tusks ; and in 

 one aberrant form {Deinotherium) the symphysial tusk-bearing 

 part of the mandible was enlarged, lengthened, and deflected. 



The bony nostril, formed by the nasals and premaxillaries, is 



