i86 . THE SKULL. [chap. 



compressed, pointed prolongation from the lower part of this 

 plate, which extends forwards in the groove of the vomer 

 ( Vo) almost to the extremity of the rostrum, and which in 

 great part remains permanently cartilaginous. This corre- 

 sponds with the septal cartilage of the nose of other Mam- 

 mals, although owing to the altered, position of the nares it 

 has here little relation with these passages. 



The cranial cavity is formed chiefly of the cerebral fossa, 

 the cerebellar fossa being relatively small, and the olfactory 

 fossa entirely wanting. 



The optic nerve passes out through a deep notch, some- 

 times a foramen, in the hinder border of the orbitosphenoid. 

 The alisphenoid is not perforated, the foramen rotundum 

 being confluent with the large sphenoidal iissure, and the 

 foramen ovale with a large infundibuliform opening between 

 the alisphenoid, parietal, exoccipital, basioccipital, and basi- 

 sphenoid, in the bottom of which is seen the inner surface 

 of the periotic {Per), which in the Cetacea makes no projec- 

 tion into the cerebral cavity. The anterior part of this 

 opening corresponds to the foramen lacerum medium with 

 the foramen ovale, the hinder part to the foramen lacerum 

 posterius.' The squamosal [Scj] appears in the outer bound- 

 ary for a very small space, between the parietal and the 

 exoccipital. The condylar foramen pierces the exoccipital, 

 near its anterior edge. The large or nearly circular carotid 

 canal has a peculiar position, passing through the basisphe- 

 noid, near its middle, in a direction from below upwards, 

 forwards, and inwards. 



The bones forming the walls of the cranial cavity are dis- 

 posed in a very remarkable manner. The occipital surface 



^ In the adult of the same species, the fornmen ovale is separated 

 from the large opening common to the seventh and eighth pair of nerves 

 by a strong bony partition formed by the ossified tentorium cerebelli. 



