234 PALEONTOLOGY 



The Mammalia are represented in a few Mesozoic 

 rocks by lower jaws and teeth of very small forms, which 

 appear to belong to primitive representatives of the two 

 orders now confined to Australasia, and there represented 

 by greatly specialized forms the egg-laying Monotremata 

 and the pouched Marsupialia. Why no other remains 

 should be found than lower jaws has never been satis- 

 factorily explained. While the Reptiles dominated the 

 world, the Mammals seem to have been a very lowly 

 and insignificant group, but on the extinction of most 

 of the former at the end of the Cretaceous period the 

 Mammals became the dominant land-animals. 



The early Eocene mammals are all of small size, with 

 the neck not flexible and the trunk passing gradually 

 into the tail. Their limbs are not greatly removed 

 in structure from the primitive five-toed (pentadactyh) 

 type, except for the presence of a projecting elbow in the 

 fore-limb and heel in the hind-limb : the two bones 

 in the second limb-segment are free to move on one 

 another so that the limbs can be rotated. There are 

 forty-four teeth in all, in continuous series, with low 

 crowns (brachydont) and a simple arrangement of conical 

 tubercles on the surface. The brain (as shown by 

 internal casts of the skull) is small like that of a reptile. 



From these primitive forms higher forms are soon 

 developed in a series of adaptative radiations, like those 

 shown by the Mesozoic reptiles. All the orders of 

 Mammalia are thus separated in the course of the 

 Eocene period. Except for the Insectivora and Rodentia, 

 which remain small, they all show a gradual increase in 



