2 I 2 Bulletin American Museum of Natural History. [Vol. XIX, 



Fig. 7. Cylindrodon /otitis. 

 Upper jaw, twice natural size, 

 No. 963g. 



from Colorado and from South Dakota it is obsolete on p4 and 

 on the molars does not extend so far down. 



? CASTORID^. 



Cylindrodon fontis Douglas. 



AYe have eight lower jaws and an upper jaw of this species, 

 some of which exhibit the tooth pattern, and enable us to 

 refer this curious little rodent provis- 

 ionally to the Beavers. The two lower 

 jaws on which the species was based 

 were of comparatively old individuals , 

 and the pattern had disappeared, so 

 that it was not possible for Mr. Douglas 

 to determine its relationship. 



The dentition is I^ c^ p| mf — four 

 cheek teeth in each jaw, as in Castoridae, instead of five above 

 and four below as in Sciuridee and Ischyromyidse. The pattern 

 resembles that of Steneofiber more than any other related form, 

 consisting in the lower molars of a 

 deep and persistent external enamel 

 inflection, and three fossettes corre- 

 sponding in position to the internal 

 enamel inflections of Steneofiber and 

 Castor. Of these fossettes the median 

 is the most persistent; the median 

 and posterior are at first internal 

 enamel inflections, the posterior in- 

 flection becoming a closed fossette 

 at a very early stage of wear, while 

 the anterior fossette is closed from 

 the first. 



It would appear from the history 

 of those teeth that the enamel in- 

 flections did not originate on the 

 sides of the tooth and become gradu- 

 ally deeper and more complicated as 



the tooth became more hypsodont, the fossettes being a 

 secondary modification; but that the inflections and fossettes 



Fig. 8. Cylindrodon fontis. 

 Crown views of lower teeth, show- 

 ing the pattern at successive stages 

 in their wear. All twice natural 

 size, Nos. 9644, 9638, 9640, 9642. 



