ZAPODIDA—DISCUSSION OF AFFINITIES. 463 
bulla, in which Zapodide recede from Muride, and more closely approach 
Dipodida, although the bulle do not attain to anything like the eyormous 
development witnessed in Dipodide. On the whole, the skull presents more 
decided affinities with that of Dipodide and Pedetide than with that of 
Muride. 
In Dipodide alone are the cervical vertebrae more or less anchylosed ; 
they are free in Mluride, Zapodida, and Pedetide. 
The elongation ef the hinder limbs of Zapodide, the shortness of the 
fore, and a correlated bulkiness of the body bebind in comparison with that 
in front, are points which, in Zapus, appear to indicate affinity with Dipodine 
forms, and which have undoubtedly been largely considered by naturalists in 
their location of Zapus near Dipus and its allies. But has not the significance 
of these features been overrated? Has not mere resemblance been taken for 
evidence of closer relationship than really exists? We should note, in the 
first place, that mere form may be fallacious; witness, for instance, the strik- 
ing contrast in shape between the closely-related Geomyide and Saccomyide. 
Secondly, although it is highly characteristic of Alurid@ to have the fore and 
hind limbs developed to approximately the same degree, yet there are some 
unchallenged forms of true Murid@, in which the hind limbs are elongated, 
such being Gerbillus and its immediate allies, collocated by Alston as a special 
subfamily (Gerbiliine) of Muride. Thirdly, it should be noted that, in spite 
of the elongation of the pes of Zapus, that member retains five perfectly 
functional digits, supported by as many complete and separate metatarsals ; 
thus falling far short of the extreme modification witnessed in Dipodida, 
where there are but three functionally developed digits, with fusion of lateral 
metatarsals. 
Upon the whole, then, while I am far from denying that the Zapodide 
are a step away from the Muride, and take that step direct toward Dipodid2,— 
as better evidenced by cranial characters than by the structure of the hind 
limbs,—still I am indisposed to concede that Zapus approaches Dipus and 
Pedetes so closely that it may be advantageously combined with these in a 
single family, the three to be contrasted collectively with the A/wride, or with 
any other one family of the Murine series. In view of the various cross- 
relationships involved, and the special combination of characters presented by 
Zapus, | continue to accord it full family rank, and consider that it is to be 
individually contrasted with Muride, as Dipodide and Pedetidz likewise are. 
