222 THE SKULL. [CHAP. 



behind which are large palatines and very small and widely 

 separated pterygoids. 



The zygomatic process of the squamosal is an immense 

 trihedral pillar in Balcena, having the large shallow glenoid 

 fossa on its under surface removed considerably from the 

 middle of the cranium, so as to give sufficient width to the 

 hinder part of the capacious mouth. The periotic and 

 tympanic are formed much on the same principle as in the 

 other sub-order, 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 processes 

 placed side by side on its front edge, to which the anterior 

 cornua are attached ; the hinder edge is excavated. In 

 Balocna the thyrohyals are cylindrical, and thicker towards 

 their free extremities. In Balanoptera musculus they are 



