270 



PROCEEDINGS OF THE ACADEMY OF 



[1895. 



shallow depression in front of the anterior one and behind the pos- 

 terior one. These teeth are all very brachyodont, with transversely 

 oval crowns, and form a slightly curved series with the convexity 

 outward, so that those of the two sides converge at both ends and are 

 most widely separated in the middle. 



It is difficult to find among modern rodents any type of dentition 

 which altogether corresponds to the one here described. The trans- 

 verse crests visible on mo of Protoptychus (and doubtless in the unworn 

 state of the other teeth, also) have a certain resemblance to the teeth of 

 the squirrels and spermophiles, but the fundamental character of the 

 tooth- pattern is given by the enamel invaginations, which tend to 

 divide it into two prisms. This arrangement is most like that found 

 in Pedetes, the Heteromyidce and Geomi/idtr. The resemblance of 

 the Uinta form to the John Day genera, Entoptyehus and Pleuroli- 



Fig. 1. Protoptychus Hateheri; 

 Skull from left side: natural 

 size. 



Fig. 2. Protoptychus Hateheri; 

 Skull from above ; natural size. 



cits, especially to the latter which is brachyodont, is decidedly 

 marked. 



The Skull. — It will be3t serve the purposes of this description to 

 consider first the skull as a whole, and then the individual elements 

 which make it up. Its general aspect suggests various affinities : on 

 the one hand, it has very marked resemblances to the skull of Dipus, 

 especially in the anterior portion, while the posterior portion strongly 

 recalls the corresponding region in Perognatlms, Crieetodipvs, and even 

 Dipodomys. The skull is quite depressed and elongate and broadens 

 less abruptly toward the hinder end than in those existing genera of 

 Dipodidce and Heteromyidce in which the mastoids are largely in- 

 flated. Seen from above, the outline of the skull has considerable 

 resemblance to that of Heteromys, though with much more prominent 

 mastoid bulhe than in the latter. In front of the orbits the face is 

 deeply constricted to form the rostrum, which, so far as it is pre- 

 served, is not unlike that of Heteromys in shape, but the proportions 



