II. MORPHOLOGY AND 

 TAXONOMY 



As already intimated, the American Mesozoic mammals fall naturally into two great 

 faunal groups, the Jurassic mammals and those of the upper Cretaceous. Even from a 

 strictly taxonomic viewpoint, this division is a natural one, for only one order, the 

 Multituberculata, is common to the two groups and even it suffered a great change in 



... Cot., 



AMF 



PMF 



B 



Fig. I. Diagram to illustrate common descriptive terms applied to the mammalian 

 mandible. A, Felis, external view. B, Felis, posterior view. C, Bettongia, posterior 

 view. D, Bettongia, inferior view of posterior part of mandible. E, Djadochta- 

 therium, inferior view of posterior part of mandible. F, Priacodon, internal view. 

 All right rami, not to scale. 



AB, alveolar border. An, angle or angular process. CAN, condylo-angular or 

 subcondylar notch. CCN, condylo-coronoid or supracondylar notch. Con, condyle. 

 Cor, coronoid process. DF, dental foramen. IG, internal groove. LB, lower bor- 

 der. MC, masseteric crest. MF, masseteric fossa. PC, pterygoid crest. PF, ptery- 

 goid fossa. PMF, posterior mental foramen. Sym, symphysis. 



the interval. Jurassic mammals are known in America only from a single formation, 

 the Morrison, which marks the close of the period.^ All of the Jurassic species known 



^ As is well known, the Morrison is almost exactly on the line between Jurassic and Cretaceous, 

 often being referred to the latter period. The writer's reasons for retaining it in the Jurassic have been 

 given elsewhere (1926F). In any event its relative position is not in question and the argument is not 

 very important. 



