ORDER MONOTREMATA. 



267 



tremata, so that if these forms be Prototheria they probably indi- 

 cate a distinct specialised order of that subclass. 



In addition to the features mentioned above, this group is charac- 

 terised by the true molars (fig. 1136) carrying longitudinal rows of 

 tubercles, separated by one or more grooves, and also by the ab- 

 sence of a pit or perforation in the masseteric form of the mandible. 



Family Plagiaulacid^e. — In the Plagiaulacidce, the premolars, 

 which vary in number from one to four in the mandible (figs. 1132- 



Fig. 1132. — A, Outer view of the right ramus of the mandible of Plagiaulax minor ; four 

 times natural size, b, Fourth lower premolar of P. Becklesi ; five and a half times natural size. 

 From the Purbeck of Dorsetshire. (After Owen.) 



1 1 38), are always of a secant nature, and are usually marked by a 

 series of oblique lateral grooves (fig. 1132); while the true molars 

 are small and reduced to two in number. In the type genus Pla- 

 giaulax of the Purbeck (Upper Jurassic) of Dorsetshire, there may 

 be either four (fig. 1132), or three lower premolars ; Professor Cope 

 regarding the latter va- 

 riation as indicating a c"V^3s^ 

 distinct genus, for which 

 he has proposed the 

 name Plioprion. The 

 upper teeth are unknown. 

 Ctenacodon (fig. 1133), 

 from the Upper Jurassic 

 of North America, is a 

 closely allied but per- 

 haps less specialised type, 

 with four lower premo- 

 lars, which are much 

 worn in the figured speci- 

 men. In the upper jaw the anterior premolars are like those of 

 Bolodon. The figure shows the relatively low position of the con- 

 dyle of the mandible in this family ; — a feature shared with the 

 Pleistocene Thylacoleo, among the Marsupials. From the Lowest 

 Eocene, both of France and North America, we have the more 

 specialised genus Neoplagiaulax (fig. 1 134), in which only the fourth 

 premolar remains in the mandible. The Puerco Eocene of North 

 America has also yielded Ptilodus (fig. 1135), characterised by the 



\ 



Fig. 1133. — Outer aspect of the right ramus of the man- 

 dible of Ctenacodon serratus ; from the Upper Jurassic of 

 North America. Upper figure, natural size ; lower, four 

 times natural size, a, Incisor; &, Condyle; c, Coronoid 

 process. (After Marsh.) 



