1895.] NATURAL, SCIENCES OF PHILADELPHIA. 277 



from weathering, it may have been present originally. If so, it must 

 have been exceedingly tine and thread-like, as it is in Dipodomys. 

 There is no postorbital ridge or plate, such as is found in Dipus, 

 though not in Zapus. The zygomatic process is longer and in every 

 way better developed than in the Heteromyidce, hut it does not pro- 

 ject out so far from the sides of the skull as in Dipus or Pedetes, but 

 has about the same degree of proportionate development as in 

 Zapus. It is separated by a considerable interval from the tym- 

 panic. 



The jugal has been largely weathered away from the specimen, 

 but the anterior portion remains, as well as the imprint of the entire 

 hone on the matrix tilling the orbit of the left side, which allows its 

 shape and connections to be determined with a fair degree of ac- 

 curacy. The horizontal portion is longer than in the recent genera 

 of Dipodidce or Heteromyidce, and though very thin and compressed, 

 forming a vertical plate, it is stouter than in any of the modern forms, 

 except Pedetes. The ascending portion is decidedly broader and is 

 continued up along the anterior edge of the orbit and forms a suture 

 with the lachrymal, as in the recent jumping-m ice. The form of this 

 process is most like that of Zapus, among the existing genera; it is 

 a wide plate, with recurved external border, convex externally and 

 concave internally, somewhat as in Zapus, though owing to the 

 smaller size of the infraorbital foramen, it is less attenuated. In 

 Dipus the edge of the plate is not recurved, hut projects directly out- 

 ward and helps to give the extraordinary breadth which the face has 

 at this point — the widest part of the skull. The anterior portion of 

 the zygomatic arch is at a lower level than in the recent jumping- 

 mice and here the arch is less horizontal; nor has it the decided 

 downward curvature found in Dipus; in front the arch is not so 

 widely separated from the alveolar portion of the maxillary as in the 

 recent genera. The zygomatic process of the maxillary is much like 

 that of Zapus. The infraorbital foramen is a large aperture per- 

 forating the zygomatic process; in shape it is high, narrow and some- 

 what pyriform, resembling that of the typical Muridce, except that 

 its position is reversed, being narrower above than below. There is 

 no separate canal, or even notch, for the nerve, such as is found in 

 the recent jumping-mice. The zygomatic process arises a little in 

 advance of the premolar, as it does also in Zapus; in Dipus it is 

 shifted somewhat farther forward, while in Pedetes it extends so far 



