1895.] NATURAL SCIENCES OF PHILADELPHIA. 271 



of the different bones which compose it resemble more the arrange- 

 ment which obtains in LHpus. The rostrum is also less tapering 

 and acuminate than in the recent Heteromyidce. The transverse 

 width of the skull is greatest in the mastoid region, though these 

 bones project but slightly beyond the line of the zygomatic arches 

 and thus the width is nearly uniform from the front of the orbits to 

 the occiput. In all of the existing Dipodidce the skull is widest across 

 the zygomatic arches. In Protoptychus the skull is long and narrow- 

 as compared with that of Dipus, though not far removed from the 

 proportions of Zap us. The interorbital constriction is very slight, 

 much as in Heteromys, and thus decidedly less than in Zapus. 

 Aside from the inflated mastoids, the broadest part of the cranium 

 is across the parietals and behind the orbits. The hinder part of the 

 cranium displays very large, rounded and swollen mastoids, such as 

 occur in the recent Dipodidce and Heteromyidce, though their shape 

 and construction are more like what we find in the latter family than 

 in the former. The mastoids encroach more upon the roof of the 

 skull and the occipital plane than in Perognathus or Cricetodipus, 

 and quite as much as in Dipus, though in a somewhat different way 

 and with a different effect upon the surrounding cranial bones from 

 the latter. 



In spite of the great size which the mastoids have already 

 attained, small, but distinct temporal fossae are preserved, such as 

 persist in Heteromys, but in no other member of that family. More of 

 the squamosal is retained on the walls of the cranium in the existing 

 Dipodidce than in Protoptychus, but not such distinctly marked 

 temporal fossie. The consistency of the bones of the skull, their 

 ridges and angles are much as in Dipus and have not that exces- 

 sively thin and papery appearance which characterizes Zapus and the 

 recent Heteromyidce. There is no indication of a sagittal crest. 

 Viewed in profile, the highest point of the skull is behind the orbits, 

 at the slightly swollen and gently convex parietals, as is also true of 

 Zapus and Perognathus] from this point the upper contour descends 

 gradually in front to the rostrum and behind to the occiput, though 

 the curvature of this surface is but slight. The inferior sur- 

 face of the cranium is rendered different in appearance from that of 

 any of the Heteromyidce by the larger size and more spherical form 

 of the tympanic bullae, which have much the same shape as in 

 Dipus, but the bullae are not separated from the neighboring cranial 



