RODENTIA. 771 



The two sub- orders of living Cetaceans the Mystacoceti, without 

 functional teeth, but with baleen-plates on the palate, and the Odonto- 

 ceti, with functional teeth and without baleen, do not seem closely 

 related, and it may be that many of their resemblances are due to 

 convergence. The toothed whales seem to be the older stock. 



The Odontoceti have probably arisen from the Zeuglodonts and 

 these from the Creodonts. Like the Sirenia, the Cetacea appeared in 

 the Lower Eocene and evolved very rapidly, attaining full adaptation 

 by the Mid-Eocene. They found the seas clear of the great Reptiles, 

 and the change from walking to floating led to many readjustments of 



FIG. 415, Vertebra, rib, and sternum of Balanoptera. 

 From specimen in Anatomical Museum, Edinburgh. 



C., Centrum ; n.a., neural arch ; n.sp., neural spine ; t.p.t transverse 

 process ; /?., rib ; St., sternum. 



a thoroughgoing sort. It is interesting to notice that flippers must 

 have arisen several times independently in Ichthyosaurs (post-Triassic), 

 Plesiosaurs, Cetacea (twice ?), Sirenia, and Pinnipedia. 



Order RODENTIA 



Rodents are represented in all parts of the world, and by 

 more species than any other order of Mammals. Most of 

 them are small and terrestrial. They are typically vegetarian, 

 and gnaw their food in a characteristic way. 



The dentition is quite distinctive. The incisors are chisel- 

 edged, for, as the enamel is either restricted to the front or 

 is at most thin posteriorly, the back part wears away more 



5 1 



