82 



parison with corresponding portions of the latter, as seen in Figs. 1,3, 4, 7, is 

 proportionately of less depth, and at the base beneath the molars curves 



much more upwardly in a backward direction. 



The teeth, represented in Figs. 10 and 11, are unworn, and they have the 

 same size and constitution as those of Hyopsodus. In the specimen the con- 

 stituent lobes of the crown appear more prominent and their intervals more 

 deeply angular than in those of Hyopsodus, but this difference is probably 

 due to difference in age. In all the specimens ascribed to Hyopsodus the 

 teeth are more or less worn, and only in that attributed to Microsus are they 

 unworn. Observing, also, that the proportionate depth of the lower jaw in 

 the different specimens of Hyopsodus holds some relationship with the age 

 of the animal, as indicated by the extent of wearing of the teeth, I have sup- 

 posed that the variations observed in the jaw-fragment of Microsus might be 

 due to the same cause, and that it therefore really pertains to Hyopsodus pau- 

 lus. Thus by comparison of Fig. 10, representing Microsus, with that of 

 Fig. 6, representing a specimen of Hyopsodus, in which the teeth are least 

 worn, the resemblance is observed to be greater than with the other and 

 older specimens of Hyopsodus. 



Comparative measurements of the specimen referred to Microsus with 

 specimens of Hyopsodus represented in Figs. 1, 3, and 6 are as follows: 



• 



Microsus. 



Hyopsodus. 





Fig. 10. 



Fig. 1. 



Fig. 3. 



Fig. G. 



Depth of lower jaw at fore part of last molar 



Antero-posterior diameter of last molar 



Lines. 



93 

 "'5 



Lines. 



H 



H 



2 



Lines. 



H 



H 



2 



Lines. 



H 



4 

 2 1 



Anteroposterior diameter of second molar 



■"5 



2 



MICROSYOPS. 



An extinct genus of small mammals, to which the above name was given, 

 was originally founded on several lower-jaw fragments containing molar teeth. 

 Though classed among the pachyderms, it is not positive that such is the true 

 position of the genus. The more complete of the specimens upon which it 

 was characterized indicates a series of six molar teeth following closely after 

 a well-developed canine. The number of incisors is unknown. 



