113 



sented in Fig. 14, Plate XXVII. It contains the last three molars, the tritu- 

 rating surfaces of which are represented in Fig. 15, magnified eight diameters. 



The jaw is proportionately deep and short, compared with that of the rat. 

 The masseteric fossa is deep, and defined by a rectangle, the apex of which 

 reaches as far forward as the position of the third molar tooth. The border 

 of the jaw at the hiatus in advance of the molars extends nearly on a level 

 from their alveoli to that of the incisor. 



The molar teeth, though having the same general constitution as the cor- 

 responding ones in the jaw-fragment of Mysops minimus, above described, 

 appear sufficiently distinct to pertain to another species, and I have therefore 

 distinguished it as such with the name of M. fraternus. 



In the jaw-specimens of both species the molars are worn nearly to the 

 same extent. In comparing the corresponding teeth, it will be seen that the 

 third molar in M. fraternus has a greater breadth fore and aft, and the last 

 molar is of more uniform width transversely. In both teeth the intermediate 

 conical lobe, occupying the inner part of the crown, is proportionately more 

 robust in M. fraternus. 



The depth of the jaw below the third molar is 2.6 lines ; the bread! h of 

 each of the three back molars fore and aft is about eight-tenths of a line; the 

 space occupied by the four molars is a little over 3 lines. 



SCIURAVUS. 



In the American Journal of Science for July, 1871, Professor Marsh has 

 described an extinct genus of rodents from remains found at Grizzly Buttes, 

 under the above name, and refers them to two species with the names of 

 Sciuravus nitidus and S. undans. The former, described from an upper-jaw 

 fragment with three molars, was about the size of the brown rat. The latter, 

 indicated by a lower-jaw fragment with the incisors and the anterior three 

 molars, was a somewhat larger animal. 



While we have not the means of determining whether Paramys is abso- 

 lutely distinct from Sciuravus, we have the opportunity of examining a speci- 

 men belonging to a different genus from the former, and which we suspect 

 pertains to the latter. The specimen iu question consists of a fragment of the 

 left side of the lower jaw, containing the third molar, the alveolus behind, and 

 part of that in front. It belonged to an animal but little larger than the rat. 

 The fossil was found at Grizzly Buttes by Dr. Carter. The only remaining 

 15 G 



