189.").] NATURAL SCIENCES OF PHILADELPHIA. 281 



the Uinta genus. The posterior part of the cranium in the only 

 known skull is so much injured that the character of the mastoids 

 cannot he made out, though they could not have been as large as in 

 Protoptychus, but the tympanies are seen to form large, hemispheri- 

 cal bullae, with very large mental openings, which do not form tubes. 

 The lateral view of the skull bears considerable resemblance to 

 that of Neotoma, as an ancestor of which Padculus is regarded by 

 ( 'ope. 



If the view as to the systematic position of Protoptychus which is 

 here advocated be correct, it follows that the Dipodidce were separ- 

 ated from the true mice at a very early period, and that they form 

 a very distinct line. It offers, however, no objection to the inclusion 

 of the jumping-mice among the Myomorpha, tor the special peculi- 

 arities of the infraorbital canal in the former are shown to be deriva- 

 tive from the murine type. The articulation of the jugal with the 

 lachrymal is probably a primitive feature, which the Dipodidce have 

 retained; this articulation is not only usual among the mammals 

 generally, but is also found in the earliest rodents, such as Paramys. 

 Another conclusion which follows from the reference of Protoptychus 

 to the ancestors of the Dipodidce is in regard to the position which 

 should be assigned to Zapus. This genus displays the skull character- 

 istics of the Dipodidce in a much less extreme degree than the other 

 existing members of the family. This is especially true of the audi- 

 tory bulla, the mastoid portion of which is very small and but little 

 inflated. If Protoptychus be really ancestral to the Dipodidce, then 

 Zapus either must have suffered a degradation and reduction of the 

 mastoid elements, or it is not related to the jumping-mice at all. 

 The first alternative is much the more probable one. This mode 

 of development, namely, the advance of a certain structure up to a 

 maximum, followed by a decline in that structure, is not an uncom- 

 mon occurrence, and is so familiar that it would be superfluous to 

 cite examples here. Such an alternation in the development of one 

 or more parts is compatible with the continually advancing ditiereu- 

 tiation of the organism as a whole. If this conclusion be well 

 founded, then Zapus is not, in any sense, a connecting link between 

 the Muridce and the Dipodidce, but a simplified member of the lat- 

 ter. We have, as yet, no means of determining how far the foot- 

 structure of Zapus retains its primitive characters, but judging from 

 the analogy of other groups, it is probable that the foot is primitive 



