MESODONTA. 225 



Comparison of the superior molars of four species which I have referred 

 to Pelycodus, with two of Hyopsodus, reveal the following characteristic dif- 

 ferences. In both species of Hyopsodus there are two distinct internal tuber- 

 cles, and there is no distinct V extending from the intermediate tvibercles. 

 That is, the posterior internal tubercle is not connected by a ridge with the 

 anterior inner tubercle. There is no internal cingulum. In Pelycodus tutus 

 and P. frugivorus, and presumably in P. jarrovii, there are two internal 

 tubercles, of which the posterior is distinct from the internal cingulum, and 

 the anterior inner tubercle is the apex of a V, which includes the interme- 

 diate tubercles. In P. angulatus and P. pelvidens, the apex of the V is the 

 only distinct internal tubercle, the anterior. The posterior is a part of a 

 cingulum which extends round the inner base of the crown. In these two 

 species there is no median external tubercle, while in the two Pelycodi first 

 mentioned the external cingulum sends up such a lobe between the exter- 

 nal cusps. The species o£ Pelycodus may be distinguished as follows: 



a. Posterior iuterual tubercle of superior molars distinct from the posterior cingu- 

 lum. 



Length of true inferior molars on base, ™.019 P. jarrovii. 



Length of true inferior molars on base, ™.017 P. tutus. 



Length of true inferior molars on base, ™.015 P. frugivorus. 



aa. Posterior internal tubercle of superior molars, small, and a process of the pos- 

 terior cingulum. 



Length of true inferior molars on base, "".024 P. pelvidens. 



Length of true inferior molars on base, "".012 P. angulatus. 



Remains of species of this genus are very common in the Wind River 

 and Big Horn bad lands. They were originally found in the Wasatch beds 

 of New Mexico, and have not yet been announced from the Bridger forma- 

 tion. 



Pelycodus pelvidens Cope. 



Lipodectes pelvidens Cope, American Naturalist, 1681, p. 1019, Nov. 29. 

 Plate XXIV tl ; fig. 3. 



The largest species of the genus, represented by a single right mandib- 

 ular ramus which supports the posterior four molars. The species is readily 

 15 c 



