434 Wortman — Studies of Eocene Mammalia in the 



donta, we shall have little difficulty in distinguishing it from 

 the Oxyclsenidse on the one hand, and the Oxysenidse on the 

 other. In the former of these families, none of the superior 

 molars are of a pronounced shearing type, but seem (according 

 to Matthew) to have been tending more in the direction of a 

 tubercular than of a sectorial structure.* From the Oxysenidse 

 the family under discussion may be distinguished by the circum- 

 stance that it is the second molar above and the third molar 

 below that were becoming the most specialized carnassials ; 

 whereas, in the Oxysenidse, it was the first molar above and 

 the second molar below that were thus specialized. In the 

 Carnassidentia, as I have pointed out on a former page (337), 

 the axis of sectorial development has shifted still further for- 

 ward, and involves the fourth premolar above and the first 

 molar below. Another important dental character of this 

 family is the comparatively small size and weak development 

 of the first molar below and frequently of the corresponding 

 tooth above as well. This is a conspicuous feature of all the 

 later species, particularly those of Ilyamodon and Pterodon, 

 and is also seen in nearly all species of Sinopa, although less 

 strongly marked. 



The progressive characters of the teeth consist in the gradual 

 loss of the internal cusps of the superior molars, the complete 

 fusion of their primary external cusps, and the loss of the 

 third molar. In the inferior molars, the heels or talons 

 decrease in size and disappear, and the internal cusp of the 

 trigon becomes rudimental or wanting. 



Sinopa Leidy. 



Sinopa Leidy, Proc. Acad. Nat. Sci., Phila., 1871, p. 115 ; 

 StypolophusCo-pe, Pal. Bull., No. 2, Proc. Amer. Philos. Soc, 1872, p. 446; 

 Prototomus Cope. Eeport on Fossil Vert. N. M., Ann. Eept. U. S. G. S. 

 W. of 100th Mer., 1874; 



Limnocyon Marsh, in part, this Journal, 1872, p. 122. 



The only genus of the family thus far known to be repre- 

 sented in the Bridger fauna is Sinopa, described by Leidy 

 from the lower part of the horizon in 1871. It first appears in 

 the beginning of the Middle Eocene or Wasatch, and is repre- 

 sented by at least six fairly distinct specific modifications in 

 these beds. One very well known species comes from the 

 Wind River beds, while at least four are now known from the 

 Bridger. The definition of the genus is as follows : 



A group of small or medium-sized Creodonts, with a complete 

 dentition, having the superior molars with well-developed internal 



* There is as yet no evidence sufficient to determine the position of this 

 group satisfactorily. They have always been assumed to be Placentals, and 

 have been placed among the Creodonts. They may quite as well be Impla- 

 centals, as far as any very good evidence to the contrary is concerned, or 

 they may prove to be Insectivores, with numerous transitional or Implacental 

 Metatherian characters. 



