NO. 1228. OSTEOLOGY OF THE JUMPING MICE— LYON. 665 



directed. * •• ""'■ There is^, a.s in Diyus^ a separate canal at tlie l)ase 

 of the foramen for the exit of the nerve." ^ 



The .skull of Sminthu.'i very closely approaches that of Zajnis, and 

 it is hard to see how Alston, in his arrangement of the llodents, could 

 have considered it as an al)erraiit memljer of the family Jfur/'<7(t'. and 

 Z(tjji(f<, Dijju^, etc., as formino- the DlpodUlw. 



The structure of the zygomatic arch and the shape of antorbital 

 foramen is almost precisely the same as are these structures in Zajnis. 

 The latter has a slightly wider malar and the separate passage for the 

 nerve is a little more marked. The palates are of the same style, but 

 the posterior free edge has a median spine in Sminthus. The only 

 reall}" striking differences are in the teeth. The upper incisors of 

 SmhitJtus are plain, and the molars (there is also a small premolar) do 

 not have the enamel in the same pattern, l)ut raised up into cusp-like 

 prominences arranged in pairs. While no skeleton is available, a 

 careful examination of the skin reveals the fact that the hind feet are 

 of similar form to those of Zajna^ — at least with respect to freedom 

 of metatarsals, number of digits and phalanges. 



Pedctes has often been classed with the Dipodldce, but recently ~ it 

 has been shown to possess many hystricomorph affinities, and Thomas 

 has placed it in that group of Rodents under the family Pedetidce. 



Dr. Coues, in Monographs of North American Kodentia, and Dr. Gill, 

 in the Arrangement of the Families of Mammalia, put Za2>a>< in a sepa- 

 rate family from that of Dipius and Alacta(j<i. It is inferred that 

 Sinlnih\i>< went to the Murkhje. It would be in strict accordance with 

 the facts, however, to associate Zapus and Sminfhn>i in one group, 

 following Winge, as the family Zapodldm; and P/jnis, AJadaga, 

 Plat //€('/•('(>/// [/f!, and Euchoreutes in an equivalent group as the family 

 Dlpodldit;'. 



The only pronounced common feature of the two families is the 

 structure of the zygomatic arch and antorbittd foramen. They all 

 present the rare condition of a lachrymo-malar articulation. The arch 

 has the most murine shape in Sminthus; Zapun is a shade further away: 

 Euchoreutes shows a condition further removed, but on a skull of 

 otherwise Dipus structure: Alactaga is much further removed, and 

 Pipus still more so from the murine form. 



The variations from a murine type of skull are entirely correlated 

 with variations from a murine type of metatarsus. The Z(pm- 

 Sminthus group with the most generalized skull has the most 

 generalized foot with the free metatarsals. We pass from general- 

 ization to specialization by both the foot and the skull and teeth from 

 Alactaga to Dipus. Similar observations hold good in the case of the 

 cervical vertebrte, 



' Sclater, Proc. Zool. Soc, 1890, pp. 610-613. 



^Thoniat^, Proc. Zool. Soc, 18%, pp. 1012-1028, and Parsons, Proc. Zool. Soc, 1898, 

 pp. 858-890. 



