496 ANATOMY OF VERTEBRATES. 



In the Ursine Seal {Arctocepltahis australis) the border of 

 the superoccipital, forming the upper part of the foramen mag- 

 num, shows the orifices of two venous sinuses. The posterior 

 border of the bony palate has an angular notch. The pterygoid 

 processes are pierced for the ectocarotids. The frontal developes 

 a superorbital plate. The mastoid projects free of the tympanic. 

 The olfactory chambers extend backward exterior to the rhinen- 

 cephalic fossa. 



In the Hooded Seal ( Cystophora cristatci), the thin basioccipital 

 shows a small vacuity. The superoccipital inclines from below 

 upward and forAvard. The temporal cristte have not met above 

 the parietals. The premaxillaries do not reach the nasals : they 

 form with the maxillaries an antorbital prominence. In the great 

 Proboscis-Seal, or Sea-Ele^^hant (Cystophora prohoscidea), the 

 occipital condyles meet upon the basioccipital : the paroccipitals 

 are less prominent than in the Cystophora cristata. Traces of the 

 suture between the basisphenoid and the basioccipital and between 

 the basisphenoid and presphenoid long remain. The entocarotid 

 canals at the back part of the petrosals are very conspicuous : 

 there are no ectocarotid canals. The sagittal crista is feebly in- 

 dicated, but the occipital crest is conspicuous for its great height 

 and thickness ; the lower border of the superoccipital j)resents 

 two vertical venous perforations, which are likemse present in the 

 Cystophora cristata. The tentorium is less ossified than in the 

 Otaria leonina. The walls of the cranium formed by the parietals 

 are thick with a coarse diploe : a very small proportion of the 

 squamosals enters into the formation of those walls : the mastoid 

 has a dense structure where it coalesces Avitli the base of the zygo- 

 matic process. Two vertical venous sinuses terminate above 

 the foramen magnum : the basioccipital is also perforated by a 

 similar venous sinus near its middle part. The petrosal is ex- 

 cavated by a deep but narrow cerebellar fossa ; a long groove or 

 notch upon its upper surface leads to the meatus auditorius 

 internus : the petrosal is, as it were, bent upwards upon this 

 groove. The tympanic bulla supports the under part of the 

 petrosal like a capsule. The tympanic cavity is divided into two 

 chambers, one above the termination of the meatus externus, the 

 other beneath and internal to it. The carotid canal perforates 

 the tympanic internal to this part of the chamber. The Eus- 

 tachian groove commences from the angle between the supra- 

 and infra-meatal divisions, and grows deeper and wider until 

 it forms the canal at the fore i^art of the tympanic bone. The 

 rhinencephalic fossa is di\^ded by a strong and sharp crista 



