NATURAL SCIENCES OF PHILADELPHIA. 305 



might not improbably belong to P. penicillatus, though he pro- 

 ceeded to make it the basis of his P. parvus. It is, however, 

 unquestionabty a Cricetodipus, as I can affirm without qualifica- 

 tion from inspection of the skull, which clearly shows the bulge 

 of the mastoid back of an occipital emargination and other cranial 

 characters diagnostic of Cricetodipus, to say nothing of the hairy 

 soles and unlobed antitragus. I therefore accept the species as 

 first clearly defined by Baird, without necessarily involving the 

 question by including the doubtful animal of Peale. 



As already suggested, the chances are that Baird was right in 

 identifying his species with that of Peale, so that the name 

 Cricetodipus parvus will stand. But should the contrary prove 

 to be the case, and Cricetodipus, Peale, 1848, be conclusively 

 shown to be a synonym of Perognathus, Maxim., 1839, anew name, 

 generic and specific, will be required for the subject of the present 

 article. The name Otognosis would be appropriate, in allusion to 

 the facility with which the species may be distinguished from 

 those of Perognathus by the structure of the ear. The present 

 species may be called 0. longimembris, as the length of the hind 

 limbs and tail is its specific character in comparison with 0. 

 Jlava. 



III. Genus DIPODOMYS, Gray. 



Dipodomys, Gray, Ann. Mag. Nat. Hist. vii. 1840, 521, type D. philhpii. 

 Macrocolus, Wagner, Abh. K. Baier. Akad. xxii. 1845, type M. halticus. 



A. Generic Characters. 



Chars, (a. cranial). Skull light, thin, papery, depressed, broad 

 behind with swollen curves, tapering in front ; rostrum acuminate, 

 produced beyond incisors ; no interorbital constriction ; palate 

 plane ; occipital surface deeply emarginate. Zygomata straight, 

 thread-like, depressed to palatal plane; abutting against t} T m- 

 panics. Anteorbital foramina represented by a circular perfora- 

 tion of the front of the maxillaries. Large excavated lachrymals. 

 Parietals triangular ; an elongate interparietal embraced between 

 forks of occipital. Squamosals reduced to small plates bounding 

 the orbits posteriorly ; other elements of the temporal bone extra- 

 ordinarily developed, thin and bladdery, their sinuses of nearly as 

 great capacity as the cerebral cavity ; the mastoids especially 

 enormous, constituting nearly all the occipital and the greater 

 part of the superior (behind the parietals) surfaces of the skull ; 



