272 PROCEEDINGS OF THE ACADEMY OF [1895. 



bones by such fissures as occur in the latter and in Pedetes, as well as 

 in Dipodoniys, conforming more in this respect to the condition of 

 Zapus. The large incisive foramina constitute another point of 

 resemblance to Dipus. 



Taking up the structure of the skull more in detail, we find that 

 the rostrum is narrow, decidelv more so than the interorbital space, 

 and quite deep vertically, its diameter in this dimension exceeding 

 the transverse. The nasals are considerably broader than in Heter- 

 omys or Perognatlius and have more the relative width found in 

 Dipus, but they extend much farther back than in any living genus 

 of either family, reaching along their external borders to the mid- 

 dle of the orbits, slightly behind the termination of the ascending 

 processes of the premaxillaries. In the Dipodidce the nasals cease 

 in front of the orbits and they extend but little more backward in the 

 Heteromyidce. The hinder ends of the nasals are deeply emarginated 

 to receive the long nasal processes of the frontals, which are far 

 more conspicuous than in either of the modern families named. 

 Posteriorly the nasals are slightly concave transversely; farther for- 

 ward they become strongly convex in the same direction and together 

 form a semicylinder. This anterior convexity is more pronounced 

 than in Dipus and it may indicate a tubular prolongation of the 

 rostrum in front of the incisors, such as occurs in the Heteromyidw. 

 The nasals are accompanied for nearly their entire length by ascend- 

 ing processes of the premaxillaries, which articulate suturally with 

 the frontals. On the top of the skull these processes are, as in the 

 Dipodidce, very narrow strips, much narrower than in the Hetero- 

 myidce. Their extension backward is more prolonged than in either 

 family, as they reach almost to the middle of the orbits. 



The frontals are short, shorter than in Zapus or Heteromys, except 

 in the median line, where the nasal processes add materially to their 

 length ; they have, however, attained no such degree of shortening 

 as is found in Dijmx. The interorbital space has about the same 

 relative width as is found in Heteromys, but the supraorbital 

 margin has not the bead- like thickening which occurs in that 

 genus. Postorbital processes are entirely wanting. The parie- 

 tals are not altogether like those of any of the modern genera which 

 have been mentioned; they are longer than in Perognatlius and have 

 not the nearly regular pentagonal shape characteristic of that genus; 

 they are somewhat shorter than in Heteromys and have, as also in the 



