498 ANATOMY OF YEETEBEATES. 



foramen jugularc. The bony tentorium terminates anterior to 

 the petrosal, wliich has an obtuse expanded inner apex, and shows 

 no petrosal pit. There is no Gasserian fossa. A ridge divides 

 the foramen ovale from the foramen rotundum. The sella turcica 

 is broad and shallow : it is defined by posterior clinoid processes : 

 there are no anterior ones. The rhinencephalic fossa is narrow, 

 but of unusual longitudinal extent ; the optic nerves traverse a 

 common canal of nearly an inch in extent before it divides. The 

 ascending plates from the palatine processes of the maxillary form 

 a deep groove for the reception of the vomer. The superior 

 turbinals occupy that part of the olfactory fossa wdiich overarches 

 the rhinencephalic chamber: this is divided by a broad crista 

 galli. A large oblong vacuity at the outer and posterior side 

 of the nasal passages between the frontal, presphenoid, pala- 

 tine and maxillary bones, is closed by membrane in the recent 

 animal. There is a smaller vacuity in the corresponding part of 

 the skulls of some other species of Seals. 



In all Seals, the convex mandibular condyle is transversely ex- 

 tended, terminal, the border of the jaw extending from below the 

 condyle forward, and rarely developing an angle : this is best 

 marked in Phoca groenlandica ; in Otaria it seems to project 

 just below the condyle. 



The hyoid arch consists of stylo- epi- and cerato-hyals, and of a 

 basi-hyal in form of a transverse bar, with a pair of thyro-hyals : 

 the stylo- hyals are attached by ligament to the outer side of the 

 petrosals. 



In the Walrus {Tricliecus rosmarus, iig. 339), the basioccipital 

 is subcarinate below. The superoccipital, 3, inclines a little up- 

 ward and forward, is di^dded by a median crista, and is bounded 

 above by a broad rugged tract. The venous fossa on the inner 

 side of the condyles is di\dded by a bony bar. There is a wide 

 sphenopalatine vacuity. The paroccipitals are broad, but not 

 very prominent : the hinder surface of the skull is much ex- 

 tended laterally by the great development of the mastoids, 8. 

 The alisphenoid is excluded from the parietal, 7, by the junction 

 of a small part of the frontal, ii, with the squamosal, 27. There 

 is no trace of a lacrymal bone, but a small elliptical canal per- 

 forates the base of the antorbital process of the frontal slightly 

 upwards. The zygomatic process of the squamosal is remarkably 

 thick. The malar sends up a lofty postorbital process, but there 

 is none on the frontal : the maxillary, 21, developes a large but 

 low sub-l)ilid antorbital process : it is perforated by a large ant- 

 orbital foramen, and excavated by a large and deep socket for the 



