20 Wort?na?i — Studies of Eocene Mammalia in the 



Discussion. — Two other species of this genus, acioleata and 

 insectivora^ were described by Cope from the Bridger, but 

 Matthew has shown"^ that the types are unrecognizable, and 

 he suggests that the names be dropped. 



That the Bridger species should be more advanced than 

 those of the TTind Kiver and Wasatch is quite natural. At 

 least three of them thus far known do not show any very great 

 advance in structure over their older representatives, but still 

 enough to separate them specifically, the remaining one, S. 

 gracilis^ exhibits very decided progress in the direction of the 

 higher specialization of the family. This is seen in the disap- 

 pearance of the posterior external cusp of the third superior 

 molar, the more decidedly connate condition of the two main 

 external cusps of the first and second superior molars, the 

 development of a more sectorial structure of the fourth pre- 

 molar, reduction in size of the heels of the lower molars, and 

 reduction of the internal cusps of the trigon, with a more 

 longitudinal shear. The internal cusps of the upper molars 

 are also reduced. 



There appears to be very good evidence of the specific 

 descent of this species. Thus, Sinopa hians of the "Wasatch, 

 as far as its structure is known, answers with tolerable accu- 

 racy to the ancestral requirements. Sinopa Whitice of the 

 intervening Wind River horizon appears to be intermediate in 

 every feature of its osteology, as far as known, with the single 

 exception that the second lower premolar is not spaced. If 

 this character were variable in the Wasatch species, it may 

 well prove to be the annectent form. 



That Hycenodon and very probably Pterodon. also, we're 

 derived from this group, there appears to be very little doubt. 

 The fundamental similarity in the structure of the skull, teeth, 

 vertebrae, pelvic girdle, limb bones, and carpus, cannot be acci- 

 dental parallelism, but on the contrary affords very strong pre- 

 sumptive proof of genetic afiinity. The most specialized 

 species of the genus, Sinopa agilis, however, cannot be in the 

 line of direct descent because of the relativel}^ small skull and 

 the sharp, compressed, and slightly fissured, bony claws ; whereas 

 in Hycenodon, at least, the skull is large in proportion to the 

 skeleton, and the claws are flat and deeply fissured. 



It should be noted here that nearly every specimen of the 

 genus Sinopa represented in the Marsh collection comes from 

 the lower part of the Bridger beds, and that not more than 

 one or two have been found in the upper part of the horizon. 

 Their complete absence from tlie IJinta, as far as we now 

 know, is, moreover, fairly satisfactoi-y proof of their extinction 



*Bull. Amer. Mus. Nat. Hist., Jan., 1901, p. 24. 



