1895.] NATURAL SCIENCES OF PHILADELPHIA. 273 



latter, a distinct ridge or angulation running along their external 

 margins, which form small temporal fossa 1 , more distinctly marked 

 than iu any other member of the family. In Dipus and Pedetes the 

 parietals are very short and wide, while in Zapus they retain much 

 the same proportions as occur in Protoptychus, though they have not 

 the external angulation, nor the concave temporal fossa? found in 

 the latter. In the fossil the parietals are slightly swollen and 

 gently convex in both directions, with a shallow depression in the 

 median line. 



The interparietal is very large and of a somewhat different shape 

 from that of any 'of the recent genera of the two families. In the 

 Heteromyinoe this bone is very wide and of varying shapes in the 

 different genera. Thus, it is elliptical in Perognathus, pentagonal in 

 Cricetodipus, and in Heteromys broadly cordate. In the Dipodo- 

 myinae this bone has become very small and is longer than wide, a 

 condition due to the great encroachments made upon the cranial 

 walls by the enormously inflated mastoids. Dipus has a broad 



Fig. 3. Protoptychus Hatcheri; Skull Fig. 4. Protoptychus Hatcheri; 



from below; natural size. Skull, occipital surface; natural 



size. 



shield-shaped interparietal, Pedetes one which is very short and wide 

 and of peculiar shape, while in Zapus it is a wide ellipse. The inter- 

 parietal of Protoptychus differs from that of all the genera named, 

 being very large and of nearly square outline, though the breadth 

 slightly exceeds the length; the latter diameter is, however, much 

 greater relatively than in any of the recent genera, while the narrow- 

 ing is obviously due to the expansion of the mastoids. The external 

 ridges of the parietals are continued over upon the borders of the 

 interparietal, which otherwise is nearly plane, but is thus made 

 slightly concave. 



The occipital surface is almost flat, except for the protuberances 

 formed by the mastoids, and is considerably higher than wide, the 

 inflated mastoid bulla? having greatly narrowed it, In general 



