ZAP0DID2E— DISCUSSION OF AFFINITIES. 463 



bullae, in which Zapodidce recede from Muridce, and more closely approach 

 Dipodidce, although the bullae do not attain to anything like the enormous 

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

 decided affinities with that of Dipodidce and Pedetklce than with that of 

 Mar idee. 



In Dipodidce alone are the cervical vertebrae more or less anchylosed ; 

 they are free in Muridce, Zapodidce, and Pedetidce. 



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

 fore, and a correlated bulkiness of the body behind 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 1 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 Geomyidce and Saccomyidce. 

 Secondly, although if is highly characteristic of Muridez to have the fore and 

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

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

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

 subfamily {Gerbiliince) of Muridce. 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 Dipodidce, 

 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 Zapodidce 

 are a step away from the Muridce, and take that step direct toward Dipodidx, — 

 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 Muridce, 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, I continue to accord it full family rank, and consider that it is to be 

 individually contrasted with Muridce, as Dipodidx and Pedetidx likewise are. 



