286 PROCEEDINGS OF THE ACADEMY OF [1895. 



mastoid bullae are filled with cancellous bone, which there is rea- 

 son to believe is a sign of degeneracy. Similar reductions of the 

 tympanic bullae are not infrequent. To give but one example out 

 of many, iu the recent peccaries the tympanies form small bullae filled 

 with cancellous hone, while in the White River ancestor of the 

 group, Perchcerus, the bullae are hollow and largely inflated. It 

 is altogether unlikely, on the other hand, that Entoptychus and 

 Pleurolicus can be ancestral to the Heteromyidce, because, in the 

 first place, the skull and skeleton are too fur advanced on the line 

 of the pocket-gophers and indicate that the burrowing habit had 

 already been more or less perfectly acquired, and, in the second 

 place, because in the imperfectly known White River genera Gym- 

 noptychus and Heliscomys we seem to find distinctly marked fore- 

 runners of the pocket-mice. 



SUMMARY. 



(I ) Protoptychus, a now rodent from the Uinta Eocene, is an unex- 

 pectedly modernized form, which has already acquired very large 

 mastoid bullae, a rostrum, incisive foramina and posterior nares 

 greatly resembling those of the jumping-mice, and, as in that 

 family, the articulation of the jugal with the lachrymal is retained. 

 The infraorbital foramen is of the murine type. The dentition and 

 the shape and construction of the mastoid and surrounding parts 

 of the cranium most resemble those of the Heteromyidce. 



i 2) The genus is probably to be regarded as the ancestral type 

 of the Dipodidce and indicates an American origin for this family, 

 being much more ancient than any known representative of the 

 group in the Old World, which it appears to have reached by a 

 comparatively late migration. Paciculus of the John Day beds is a 

 somewhat aberrant number of the same line. 



(3) It is not improbable that the Heteromyidce were derived from 

 some form related to Protoptychus, though not from that genus itself. 



( I ) The Geomyidce are descended from early forms which may 

 West he referred to the Heteromyidce and in which the tympanies 

 and the mastoids were already greatly inflated. The assumption of 

 subterranean habits of life brought about a reduction in this region 

 of the skull and led to the acquisition of the many peculiarities 

 which characterize the recent pocket-gophers. Pleurolicus and 

 Entoptychus represent stages in this change and are more or less 

 directly ancestral to the modern Geomyidce. 



