1896.] Geology and Paleontology. 133 



The question now arises what is Pttrhtmolophus, or in other words 

 what is the exact generic definition of this genus, and does it really 

 occur in the Wasatch of America. It is usually stated that Parhyiw- 

 lophus is separated from Hymcotherium hy the fact, that the last pre- 



definition will apply to only one European species, namely: Pitchy nolo- 

 phus (=Hyracothe,iin,i - sidrmHthints, and even in this species the last 

 upper premolar exhibits a good deal of variation in its structure. 

 Kiitimeyer' figures Pietot's species, /•*. sideroiit hints, which in this spec- 

 imen has the last upper premolar molariform, but in the same plate 

 (fig. 21) is given another last upper premolar, which Rutitneyer referred 

 to the same species. This tooth is tritubercular, or simpler in structure 

 than the true molars. 



Among the specimen.- of Parhy)i,d»/,h us in the collection of the Jar- 

 din des Plantes, Paris, there is a series of loose teeth from theSiderolithie 

 du Mauremont, which are of considerable interest as they were studied 

 by Kowalevsky, and referred by him to P. siderolithieas. This series 

 contains at least one last upper premolar, and it has exactly the same 

 character as that figured l>y liutimeyer, in other words this is another 

 case in which this tooth in P. sider<dithinis is simple in structure. In 

 Patdtiintdttphti.-- thmt/ii a- 'per premolar 



is tritubercular, and this tooth has the same structure in P. demaratU. 

 In Pachynolophus cessarasicus figured by FilhoP from the Middle Eocene 

 of (essai. the last upper premolar has only one internal cone and the 

 two transverse ridges diverging from it are well developed. The length 

 of the skull in this species is about one-third greater than that of Hyra- 

 eothrrlum renticolum of the Wasatch. 



Numerous species of Pacht/htdo/ditts occur in the different horizons of 

 the Eocene of France, but in nearly all cases, they are represented 

 either by upper or lower molars which were not found together. I 

 believe the English Hyracothrriniu hp<,rinum is the only known form 

 in Europe in which both upper and lower molars were found associated, 

 and actually belong to the same individual. 



As is well known it is the last lower premolar which first becomes 

 molariform, consequently if we find forms in which this tooth is simpler 

 in structure than the molars, we can safely conclude that none of the 

 superior premolars are molariform. In a jaw referred to Pachynolophus 

 in the collection of the Jardin des Plantes from Phosphorites, contain- 

 ing all the lower premolars, the last tooth of this series is not molari- 



5 EocuneSangethier^ Well 



