xii.] CETACEA. 215 



and posteriorly reaches for a considerable distance beneath 

 the basisphenoid. It forms as usual the inner wall of the 

 posterior narial apertures. Behind these apertures the base 

 of the skull is flat in the middle line, but with prominent 

 lateral elevations formed by the basioccipital, continuing 

 the pterygoid ridge backwards. The glenoid fossa (gf) is 

 a shallow, oval facet, on the inner and under surface of the 

 zygomatic process of the squamosal. 



The periotic region of the skull differs much from that 

 of most Mammals. On the side of the base of the cranium 

 is a large recess, bounded below by the prominent edge of 

 the basioccipital on the inner side, by a projecting edge of 

 the exoccipital (paroccipital process) behind, by the base 

 of the zygomatic process of the squamosal externally, and 

 by a long curved process from the same bone in front, and 

 communicating with the cranial cavity above by an irregular 

 opening between the exoccipital and the alisphenoid. In 

 this recess lies a bone of singular shape which, having only 

 a ligamentous connection with the surrounding bones, is 

 easily separated from the rest of the cranium in maceration, 

 and is hence often wanted in specimens in museums. This 

 is the united tympanic and periotic, ankylosed in the adult, 

 but in young specimens still separable into its two com- 

 ponent parts. 



The tympanic (Ty) is a hollow, bullate bone, broad, 

 rounded and bilobate behind, and pointed in front. It 

 is open above, the hinder part being, however, in relation 

 with the periotic. Through the anterior spout-like end the 

 Eustachian canal passes. At the upper border of the outer 

 side, rather behind the middle, is an irregular or somewhat 

 crescentic opening, bounded in front by a prominent lip, 

 this is the meatus auditorius externus, closed in the living 

 animal by the membrana tympani. 



