ZAPODIDiE— DISCUSSION OF AFFINITIES. 



4G3 



bullse, in which ZapodidiB recede from Muridie, and more closely npproach 

 Dipodida, although the bullje do not attain to anything like ihc enormous 

 development witnessed in DipodidcB. On the whole, the skull presents more 

 decided affinities vith that of DipodidcB and PedetidcB than with that of 

 Muridce. 



In DipodidcB alone arc the cervical vertebrae more or less anchyloscd ; 

 tlicy arc free in Murida, ZapodidtB, and Pedetidte. 



The elongation of the hinder limbs of Zapodidce, the shortness of the 

 fore, and a correlated bulkincss of the body behind in comparison with that 

 in front, are points which, in Zapus, appear to indicate affinity with Dipodinc 

 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 Gfomyidee and Saccomyidce. 

 Secondly, although it is highly characteristic of Murida to have the fore and 

 hind limbs developed to approximately the same degree, yet there are some 

 unchallenged forms of true Murida, in which the hind limbs are elongated, 

 such being Gerbillus and its immediate allies, collocated by Alston as a special 

 subfamily {Gerbiliinai) of Murida. 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 Dipodider, 

 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 Zapodida 

 are a step away from the Muridte, and take that step direct toward Pipodidx, — 

 as better evidenced by cranial characters than by the structure of tiio 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 Murida, or with 

 any other one family of the Murine series. In view of tlie various cross- 

 relationships involved, and the special combination of characters iircsented by 

 Zapus, I continue to accord it full family rank, and consider that it is to bo 

 individually controstcd with Muridec, as Dipodid.2 and Pedetidx likewise are. 



