216 TBE WASATCai AND BRIDGER FAHN^. 



Last premolar with inner cusp; all the molars with basin- 

 shaped heel behind ; second premolar one-rooted Tomitherium. 



Like Tvmitherium, but second i)rcniolar two-rooted Pelycodus. 



Last i)reniolars with inner cusp; all the molars with elevated 



cusps behind ; second premolar two-rooted Sarcolemttr. 



AA. The anterior inner cusp undivided on all the molars. 



Last molar with heel; last premolar with inner cusp; four pre- 

 molars Hyopsodus. 



Last molar without heel. 



Premolars four, last large ; sectorial, without inner cusp Aphelucns. 



Three or two premolars ; last without inner cusp Anaptomorphua. 



U. Last molar with a lonfritudinal series of alternating cusps, including a heel. 



Tubercles of molars 1-2, alternating Adapis. 



Tubercles of molars 1-2, opposite Opistlwtomus. 



MICROSYOPS Leidy. 



Proceed. Acad. Phila. 1872, p. 20 (charncterized). Report U. S. Geol. Surv. Terrs, i, p. 82, 1873. Cope, 

 Eeport Expl. and Sun-. W. of lOOtli Mer., G. M. Wheeler, iv, pt. ii, p. 134, 1877. 



This genus is easily distinguished from its alhes by the absence of the 

 fii'st inferior premolar, and probably of the superior first premolar also. 

 The canine tooth of the lower jaw of the M. scoitianus and M. gracilis is 

 very large, so that I suspect that when the dentition is fully known there 

 will be found to be a deficiency in the number of the inferior incisors. The 

 dentition is otherwise as in Sarcohmur, i. e., the last premolar with internal 

 cusps, and two anterior internal cusps of the true molars. 



The three species are distinguished as follows : 



I. Heel of last inferior molar very short. 



Length of true molar series '".0077 M. npierianm. 



Length of true molar series "'.0172 Jf. gracilin. 



II. Heel of last inferior molar long. 



Length of true molar series ™.0135 M. scotiianus. 



MiCROSYOPS SPIERIANUS Cope. 



Americau Naturalist, 1879, p. 908. 



Platr xxvn.iig. 8. 



Established on a portion of a nuuidibular ramus which contains the 

 three true mola is in perfect preservation. As the number of premolar teeth 

 is unknown, its reference to this genus is provisional only. The last true 

 molar has the form of that of the M. gracilis. It is distinguished by its 

 small size, since it is considerably less than the Hyopsodus vicarius {HJ 



