O. C. Marsh — Cretaceous Mammalia. 85 



The specimens known indicate an animal nearly the size of a 

 rat. They were found in the Laramie of Wyoming. 



Na7iomys minutus, gen. et sp. nov. 



A fourth genus allied to those above described is indicated by 

 some very minute teeth, one of which, selected as the type, is 

 shown on Plate II, figures 9-12, three times natural size. It 

 is apparently the last upper molar of the left side. The 

 composition of the crown is peculiar in having two rows of 

 cones separated from each other by a longitudinal depression 

 which is not a straight groove, but irregular in direction. On 

 what appears to be the outer side, there are eight cones, the 

 the anterior four being large, and the posterior ones, quite 

 small. On the opposite side, there are two large cones in 

 front, and five minute tubercles behind them. 



The remains representing this diminutive species were found 

 in Wyoming, in the Laramie formation. 



Dipriodon robustus, gen. et sp. nov. 



The present genus has a form of dentition not before seen. 

 This is represented in the type specimen shown on Plate II, 

 figures 13-15. This specimen is the last upper molar of the 

 left side, its position being decided by a portion of the maxillary 

 attached to it. The composition of the crown of this tooth 

 consists of two rows of cones, separated by a deep longitudinal 

 groove. The elevations in each series are crescentic in form, 

 the convex portion of the crescent being forward. There are 

 three cones in the outer row, and but two in the inner row. 

 Their points are somewhat worn, apparently most by a lateral 

 motion of the jaws. 



A second isolated last upper molar, evidently of the same 

 species, was found at another locality in the same horizon. It 

 agrees in all essential characters with the type specimen figured. 



A smaller species of the same genus is indicated by the 

 upper molar tooth represented on the same plate, figures 16-18. 

 This tooth is apparently a first or second molar, also from the 

 left side. It has four cones in the outer row, and three in the 

 inner. This species may be called Dipriodon lunatus. 



The animal represented by the larger specimen was about as 

 large as a rabbit, and the other, somewhat smaller. The type 

 specimens are from the Laramie of Wyoming. Various parts 

 of a skeleton were found with these remains, some of which 

 are figured on Plate V. 



The family represented by the two species may be named 

 the Dipriodontidve. 



