14 



Ctenacodon and its allies 

 Upper premolars 5, simpler. 



P*"^ shearing. 



Lower premolars 3-4, P4 with 5-8 



AMERICAN MESOZOIC MAMMALIA 



Ptilodtis and its allies. 



1. Upper premolars 4-3, may be more 

 complex. 



2. Only last upper premolar shearing. 



3. Lower premolars 1-2, P4 with 6-14 

 serrations. 



4. M} much larger than M|. 



5. M^"^ with three rows of cusps. 



6. Molar cusps more numerous. 



7. Molar cusps more or less crescentic 

 and generally complicated by other 

 ridges and grooves. 



2. 



3- 

 serrations. 



4. Ml and M| subequal. 



5. M^'^ with two rows of cusps. 



6. 2-4 cusps in each row on molars. 



7. Molar cusps simple and nearly coni- 

 cal. 



Fig. 3. Diagram illustrating evolution of plagiaulacoid molars. A-D, transverse sections of molars 

 in occlusion. A, Plagiaulacidae. B, Ptilodontidae, more primitive stage. C, Ptilodontidae, more 

 advanced stage. D, Taeniolabididae (somewhat worn). E-G, crown views of left upper molars. E, 

 Ctenacodon. F, Cimolomys. G, Taeniolabis. Not to scale. 



As has long been held, these differences are mainly such as could be imputed to 

 evolutionary advance in the interval between the earliest Cretaceous and the upper 

 Cretaceous. Nevertheless it is apparent that the morphological break is great and that 

 the differences are at least as marked as those used to separate allied families among 

 later mammals. Such a feature as the presence of two subequal upper shearing teeth in 

 the earlier forms and only one enlarged shearing tooth preceded by smaller grasping 

 teeth in the later ones may indicate that the relationship is not a directly ancestral one. 

 In any event our present knowledge requires their separation into two families. For 

 the earlier group the name Plagiaulacidae, in a restricted sense, is retained. The later 

 group is named Ptilodontidae, after its best known and most typical genus. 



In the present section we are concerned only with the American Plagiaulacidae. 

 The Mesozoic Ptilodontidae will be discussed below in treating the faunas of the upper 

 Cretaceous. 



PLAGIAULACIDAE Gill 1872 



Definition. — Premolars five above and four or three below, the lower premolars 

 shearing inside the subequal last two upper ones. Molars, upper and lower, all of about 



