RODENTIA. 77% 
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 appearéd 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 
Fic. 415.—Vertebra, rib, and sternum of Balenoptera.— 
From specimen in Anatomical Museum, Edinburgh. 
C., Centrum; 2.@., neural arch ; #.s5/., neural spine ; 4.2., transverse 
process; #., rib; S%., 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 RopENTIA 
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. 
Thedentition 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 
