308 A BOOK OF -WHALES 



process has not been fully completed, and there are 

 small nasals which just project over the nasal 

 vacuities. The symphysis of the mandible of 

 Squalodon is very long, thus recalling the Platanistids 

 and Physeter. 



These whales, which did not exceed some thirty 

 feet in length, have been divided into numerous 

 genera ; but as little is known of the skeleton this 

 proceeding is at present rather premature. Cope, 

 however, allows another genus in addition to those 

 mentioned, and that is Trirhizodon, characterised by 

 the fact that some of the molars are three-rooted. 



ARCH&OCETI 



This, the last of the three divisions of whales, 

 embraces only a single family, and, so far as can be 

 said with certainty, only a single genus, Zeuglodon. 



It is usually regarded as an assemblage equivalent 

 to either of the other groups, and this view will be 

 followed here. But the differences in structure might 

 fairly be considered as entitling it to a more isolated 

 position among the Cetacea. Nevertheless, there is 

 no question of the Cetaceous nature of Zeuglodon. 

 It is quite possible, however, that the Zeuglodonts 

 are the ancestral group from which both Odontoceti 

 and Mystacoceti have been derived. But this view, 

 a very general one, cannot be elaborated in detail ; 

 we shall simply find an example of what is so dis- 

 appointingly general when an attempt is made to 

 trace pedigrees in animals. 



