660 PROCEEDINGS OF THE NA TIONAL MUSEUM. vol. xxiii. 



esses. Four vertebrj^ form the sacrum, which is of the same form 

 in each, and like that of the 2Iu/ida\ The caudal series is much 

 long-er than all the preceding portions of the column taken together; 

 Zajms has the greater number of vertebrje — about thirty -six (there is 

 some variation in different skeletons) — and the skeleton of Dijnis 

 shows twenty-eight. 



The atlas is a large ring, essentially the same in each genus, but the 

 second cervical or axis shoAvs considerable differences in the two 

 genera. 



In Zapus it is well developed and en- In i»//>«.s the axis and the fiuir sncceed- 



tirely free from the remaining five dis- ing vertebne are completely fused into 

 tinct vertebrae behind, as well as from one large comi^ound "axis," with a large 

 the axis in front. compound neural spine which shows no 



signs of segmentation. The fused cen- 

 trum does show signs of segmentation, 

 however. 

 In Ziipus atlas and axis are entirely free In Dipus atlas and axis, free dorsally 

 and articulate in the usual manner. and laterally only, below they are fused 



into one piece. 



The seventh cervical is free from the rest of the series in each case. 

 The pectoral arch presents a few differences. 



In Zapns the clavicle is longer, slen- In Dipm, clavicle stouter and heavier 



derer, and uniformly curved, convex out- and somewhat in the shape of an italic.?', 

 wardly. 



Scapula with the vertebral border cur\- Scapula with an almost straight verte- 

 ing into the anterior border, a shape usu- bral border, which does not slope gradu- 

 ally seen in the Muridw. The supraspi- ally into the anterior border. The su- 

 nous and infraspinous fossa? are about praspinous is much smaller than the in- 

 equal in size. fraspinous fossa. 



The anterior limbs present no noticeable differences aside from rela- 

 tive proportions. 



The fore limb of Zapus is about one- The fore limb of Dipus is about one- 

 half the hind limb, or about three-quar- quarter the hind limb, or about two- 

 ters of the dorso-lumbar series of verte- thirds of the dorso-lumbar series of verte- 

 bra?, bra?. 



The pelvis shows no differences. 



The hind limbs show marked differences, both as regards relative 

 size and the number of elements in them. The ratio of the lengths of 

 the different segments of the limbs to the total length of the leg is 

 practicalh^ the same in each, but — 



Zapus has shorter legs, the dorso-lum- Dipufi has longer legs, the dorso-lumbar 

 bar series of vertebne being about two- series of vertebra? being about one-third 

 thirds the length of the hind limb. the total length of the limb. 



The femur is similar in each, Init Zajxis has a triangular projection 

 (third trochanter) on the upper outer side, which, commonly found in 

 the 2IurkI(e, is lacking in Dipus. 



