MAMMALIA. 



The living Cetaceans are ranked in two sub-orders the 

 Mystacoceti or Balaenoidea, without functional teeth, but 

 with whalebone or baleen-plates on the palate, and the 

 Odontoceti or Delphinoidea, with functional teeth and 

 without baleen. 



Certain Eocene fossils, known as Zeuglodonts, are regarded by some 

 (Lydekker, Dames) as primitive Cetaceans Archceoceti - less special- 

 ised than modern forms, but Professor D'Arcy Thompson has advanced 

 strong arguments in favour of their affinities with Pinniped Carnivores. 



I 

 1 iR. 



FIG. 370. Vertebra, rib, and sternum of Balcsnoptera. 

 From specimen in Anatomical Museum, Edinburgh. 



C.j Centrum ; n.a., neural arch ; ./., neural spine ; /./., transverse 

 process; R., rib; St., sternum 



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 characteristic. 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 

 rapidly. The incisors are rootless, growing from persistent 

 pulps, and the same is sometimes true of the bunodont or 

 lophodont back teeth. There is never more than a pair of 



