POUCHED ANIMALS. 



149 



small single molars, found in the uppermost strata of 

 the Keuper formation in England and Wiirtemberg, and 

 which are the oldest known vertebrate remains, probably 

 belong to these primseval Promammalia. These teeth, by 

 their form, indicate species that lived on insects ; the species 

 has been called Microlestes antiquus. Teeth belonging to 

 another closely allied Primitive Mammal {Dromatlieriutn 

 silvestre) have recently been discovered in the North 

 American Trias. 



On the one hand, the still extant Beaked Animals, and, on 

 the other, the parent-forms of the Pouched Animals (Mar- 

 supialia, or Didelphia), must be regarded as representing 

 two distinct and divergent lines of descent from the Pro- 

 mammalia. This second Mammalian sub-class is very 

 interesting as a perfect link between the two other sub- 

 classes. While the Pouched Animals, on the one side, retain 

 many of the characters of the Cloaeal Animals, they also, 

 on the other, possess many placental characters. A few 

 characters are quite peculiar to Pouched Animals alone ; 

 such, for instance, is the structure of the male and female 

 sexual organs, and the form of the lower jaw The dis- 

 tinctive feature of the latter in these Pouched Animals is a 

 peculiar hook-shaped bony process, passing inward hori- 

 zontally from the angle of the lower jaw. As neither 

 Cloaeal Animals nor Placental Animals have this process, 

 tliis structure is alone sufficient to distinguish the Pouched 

 Animals {Marsuinalia). Nearly all the known mammalian 

 fossils from the Jurassic and Cretaceous formation are lower 

 jaws. Our whole knowledge of numerous mesolithic mam- 

 malia, the former existence of which would otherwise never 

 have been known, is solely derived from their fossilized 



