300 Bulletin American Museum of Natural History. [Vol. XVI, 



our specimen is no trace of external cement. This specimen 

 comes from the base of the Loup Fork beds at Courthouse 

 Butte, near Pawnee Buttes, Colorado. 



Dimensions, No. Qj6i. 



Anteroposterior diameter of lower molar 7.4 mm. 



Transverse " " " " 4 



Length of diastema 7 



Transverse width of incisor 3 



Anteroposterior diameter of incisor 4 



CASTORID^. 



Steneofiber Geoffroy. 



The pattern of the molars in this genus is so evanescent 

 that it is almost impossible to make satisfactory comparisons 

 of species on the limited number of specimens available. The 

 essential pattern of the upper teeth consists of a deep internal 

 enamel inflection and three external ones, the anterior and 

 posterior of which quickly become fossettes. In the lower 

 teeth the pattern consists of a simple external and three in- 

 ternal inflections, the anterior and posterior internal inflec- 

 tions soon becoming fossettes. With moderate wear the 

 crown is divided transversely, by the internal and external in- 

 flections, into an anterior and a posterior column, united at the 

 base, each column containing a fossette, sometimes more than 

 one. The teeth become broader transversely with wear, and 

 much less in their anteroposterior diameter; the anterior col- 

 umn increases and the posterior one diminishes in size in the 

 upper teeth, while the converse holds true in the lower teeth. 

 The enamel folds become closer, and their direction changes with 

 wear; the internal one in the upper teeth, the external one in 

 the lower, becoming more nearly anteroposterior in direction. 

 The principal inflections in old individuals have become fos- 

 settes, while the minor inflections have disappeared. The last 

 molar in young individuals appears small in proportion, because 

 of the small area of the grinding surface ; in older individuals 

 it appears larger in proportion. The fourth premolar of 



