1895.] NATURAL SCIENCES OF PHILADELPHIA. 279 



The new genus and species may he defined as follows: 



PKOTOPTYCHUS. 

 Gen. nov. p : , in L, cheek-teeth brachyodont, in the worn 

 condition with deep enamel invagination from outer side and 

 shallow depression on inner side of crown. Infraorbital foramen 

 large, pyriform, perforating zygomatic process of maxillary. Jngal 

 articulating with lachrymal (?). Mastoids much inflated, in degree 

 intermediate between those of Cricetodipus and Dipodomys; tym- 

 panies much inflated and hemispherical. Interparietal nearly square. 

 Incisive foramina very large. 



P. Hatcheri n. sp. 



Length of skull about as in Perodipus Ordi. Internal enamel 

 invagination of p i much less marked than in m grinding teeth 

 of nearly equal size, m _L slightly larger than the others. Nasals 

 wide. Occiput higher than wide. 



This species is dedicated to its discoverer, Mr. J. B. Hatcher, 

 whose long and arduous labors in behalf of palaeontology have 

 been crowned with such brilliant success. 



Length of skull from inion 

 to zyg. proc. of max. 



Breadth of skull at mas- 

 toids 



Breadth of skull across 

 zygomatic arches 



Breadth of skull at inter- 

 orbital space 



Width of rostrum at base .007 



The Affinities of Protoptychus. 



It has been remarked that fossils do not aid materially in clearing 

 away morphological difficulties, because they raise as many problems 

 as they solve. Such a stricture, while not justified in the case of a 

 nearly complete phylogenetic series, is only too applicable to isolated 

 genera, such as Eloiheriwn, or in the case of a few scattered and 

 widely separated links in a phyletic chain. To the latter class, un- 

 fortunately Protoptychus belongs and there is much about it that is 

 extremely puzzling. It is, in the first place, altogether unexpected 

 to find that the curious and highly specialized family of the Dipodidoe 

 should extend back with so little change to the upper Eocene. This 



