194 ^-^^ SKULL. [CHAP. 



involuted to form an outer wall to the post-palatine air sinus. 

 In some of the Ziphiifice the rostrum is very dense, the 

 anterior prolongation of the mesethmoid being either 

 partially or completely ossified, and ankylosed with the 

 surrounding bones. 



In Hyperoodon., the crest at the vertex is high and mas- 

 sive, being formed by the nasals, the ascending plates of the 

 premaxillae and maxillae, the frontals and supraoccipital. 

 Separated from this crest by a depression, there is on each 

 maxilla, at the commencement of the rostral portion of the 

 skull, a very thick and high longitudinal ridge. 



An easy transition from this cranium leads to that of 

 the great Sperm Whale or Cachalot {Physeterviacrocephalns)^ 

 which of all Mammals is perhaps the most modified from 

 the ordinary type. The transverse vertical crest and the 

 longitudinal maxillary crests are united, to form the walls 

 of a great semicircular basin, surmounting the whole of 

 the back part of the cranium, open only above and in 

 front. The bones composing this wall are the same as in 

 Hyperoodon, but excessively expanded and flattened. The 

 rostrum is broad at the base, gradually narrowing to the 

 front, and immensely elongated. The great supracranial 

 cavity lodges the oily substance which, when refined, is 

 known as spermaceti. 



The skull of the Cachalot is remarkable for its want of 

 symmetry, especially in the region of the anterior narial 

 apertures, of which the left is very much larger than the 

 right In consequence of the small increase in the size of 

 the brain cavity, compared with that of the external parts of 

 the head in these enormous animals, the foramina through 

 which the nerves pass out of the lateral parts of the base 

 of the skull, are long channels excavated through immense 

 bony masses. The petro-tympanic bone, which is scarcely 



