MULTITUBERCULATA. 251 



ridges on the outer face (hence the name Plagriautacodon, abbreviated to 

 Plagiaulax). Nothing is known of the mode of succession, but these 

 teeth are termed premolars. Finally, there are two relatively small 

 molars, of which the crown is hollowed in the middle and bears tubercles 

 round the edge. The typical species is P. becklesi, from the Middle 

 Purbeck beds of Durdlestone Bay, Swanage, with mandible attaining a 

 length of not more than 0'03 m. 



Bolodon. An upper jaw from the same stratum and locality as 

 Plagiaulax, and quite possibly referable to this genus. The pre maxilla is 

 short and bears at least two incisors, both a little notched and the first 

 about twice as large as the second. There is a short diastema at the 

 anterior end of the maxilla. Then follow three premolariform teeth, each 

 apparently tritubercular; and finally there seem to have been four molars, 

 all bearing a double antero-posterior series of tubercles. 



Gtenacodon (fig. 147). Mandible as in Plagiaulax, but the four pre- 

 molars smooth on the outer face, only notched at the cutting edge. An 

 imperfect maxilla ascribed to this genus on evidence not stated (fig. 

 147 A, B) exhibits teeth much like those of Bolodon. The known species 

 are as small as those of Plagiaulax, and occur in the Upper Jurassic of 

 Wyoming, U.S.A. 



Fio. 148. 



Ptilodus mtdicevus; imperfect left mandibular ramus, from the outer (a) and 

 inner (6) aspects, nat. size, and from the oral aspect (c) f nat. size. L. 

 Eocene (U. Puerco); New Mexico. (After Cope.) 



Detached teeth and fragments of jaws apparently identical 

 with those of the Plagiaulacida? have also been found in the 

 uppermost Cretaceous (Laramie Formation) of Wyoming. Only 

 the third and fourth premolars have been observed in the 

 lower jaw, while the third is relatively small with a single 



