198 THE SKULL. [chap. 



hinder part of the capacious mouth. The periotic and 

 tympanic are formed much on the same principle as in the 

 other suborder, and in adult animals are completely excluded 

 by a considerable distance from the cranial cavity, owing to 

 the thickness of its walls. Instead of the small flattened 

 tongue-shaped process projecting backwards from these bones, 

 there is a long pyramidal tenon-like process, which fits into 

 a groove in the squamosal and appears on the external sur- 

 face of the skull like, though more solid than, that of the 

 Cachalot. In addition to this another process projects out- 

 wards and backwards, and the two together hold the bones 

 much more firmly in their place than in the Toothed Whales. 

 The tympanohyal is a large conical bony mass, with a 

 truncated base, with which the stylohyal is connected, and 

 firmly ankylosed by its apex to the periotic. 



The mandible differs much from that of the Toothed 

 Whales. The two rami of which it is composed are not com- 

 pressed and straight, but rounded and arched outwards, and 

 never have extensive, flat, opposed symphysial surfaces, but, 

 curving towards each other, meet at an angle in front, where 

 they are held together by strong bands of fibrous tissue. 



The hyoid arch is formed essentially on the same plan as 

 in the other Cetacea. The basihyal has a pair of pro- 

 cesses placed side by side on its front edge, to which 

 the anterior cornua are attached ; the hinder edge is exca- 

 vated. In Balccna the thyrohyals are cylindrical, and thicker 

 towards their free extremities. In Balcenopte7'a musculus they 

 are cylindrical and tapering, in B. 7'ostrata, flat and pointed 

 externally. They always ankylose with the basihyal. 



Order Sirenia. — The animals belonging to this order, 

 restricted at the present time to only two genera, which were 

 ormerly, but quite erroneously, included among the Cetacea, 



