81 



Hyopsodus minusculus. 



A smaller species of Hj'opsodus than the one described in the preceding 

 pages appears to be indicated by a specimen discovered by Dr. Carter in the 

 buttes of Dry Creek. The specimen consists of an Tipper-jaw fragment con- 

 taining the true molars and part of the last premolar, which are represented 

 in Fig. 5, Plate XXVII. The teeth differ in no essential character and only 

 in size. Their comparative measurement with those of H. paulus are as 

 follows : 



Length of space occupied by the last premolar and molars 



Length of space occupied by the molars 



Breadth of last premolar 



Width of last premolar , 



Breadth of first molar 



Width of first molar 



Breadth of second molar 



Width of second molar 



Breadth of third molar 



Width of third molar 



Hyopsodus 



Hyopsodus 



niinuscnlus. 



paulus. 



Lines. 



Lines. 



5.4 



G.45 



4.4 



5.2 



1.05 



1.4 



1.8 



2.2 



1.6 



1.8 



2.0 



2.4 



1.6 



2.0 



2.1 



2.8 • 



1.25 



1.6 



1.7 



2.35 



MICROSUS. 



MlCROSUS CUSP1DA-TUS. 



The genus Microsus is obscurely determined and is uncertain in its distinc- 

 tion from the previous genus. It was originally inferred from the lower-jaw 

 fragment with the back two molars, represented in Fig. 10, Plate VI, at the 

 same time that Hyopsodus paulus was characterized from the only specimen 

 then in our possession, represented in Fig. 1 of the same plate. The well- 

 marked difference in the form and proportion of the corresponding portion of 

 the jaw led me to view it as pertaining to a different genus from Hyopsodus. 

 Subsequently I have had the opportunity of examining many and more char- 

 acteristic specimens referable to the latter, but none which with any certainty 

 could be ascribed to Microsus. 



The jaw-specimen referred to the latter was obtained by Professor Hay- 

 den on Black's Fork of Green River. The jaw would appear to be narrower 

 and weaker than in Hyopsodus. The fragment as seen in Fig. 10, in com- 

 11 G 



