144 bulletin: museum of comparative zoology. 



Following are measurements of the skull and other bones of C. nana. 

 Skull: — greatest length 51 mm.; basal length 45.5; palatal length 

 26.4; diastema 13.7; nasals 15; zygomatic width 28; mastoid width, 

 20; least interorbital width 11.7; width outside last upper molar 9; 

 audital bulla 12 X 8.6; vertical diameter of orbit 11.8; of antorbital 

 foramen 9.5; length of upper tooth-row 11; of lower tooth-row 11.2. 

 Length of humerus 32.7; radius 29.5; ulna 36.7; femur 39; tibia 

 40.5; fibula 36; pelvis 45.5. 



The number of vertebrae in the four species of the genus, and in a 

 specimen of Geocapromys, was found to be as follows : — 



Cervicals Dorsals Lumbar s Sacrals Caudals 



The agreement in total number of vertebrae between C. prehensilis 

 and C. melanurus is probably evidence of their close relationship. 

 In the only two skeletons examined, that of the former had one more 

 dorsal but one less lumbar than that of the latter. In C. nana, the 

 number of ribs is slightly reduced — sixteen — and there are but six 

 lumbars. Geocapromys has the fewest ribs — fifteen. The tail- 

 vertebrae are 28 in C. prehensilis, C. melanurus, and C. nana, but are 

 more reduced in C. pilorides (with 21) and Geocapromys with only 

 seventeen. 



The structure of the sternum is essentially similar in all four species. 

 In C. pilorides, there are in addition to manubrium and xiphisternum, 

 six other sternebra. In a ventral view, the adult sternum shows the 

 fifth and sixth practically fused, though the line of fusion is distinctly 

 marked. The seventh sternebrum is visible in a dorsal view only, as a 

 very short nodule of bone, lying above the posterior end of the fifth. 

 It offers attachment to the cartilaginous portions of the seventh pair 

 of ribs. The xiphisternum adjoins this segment and consequently 

 lies dorsal to the plane of the others. The cartilaginous parts of the 

 eighth pair of ribs alone unite with its anterior end. The conditions 

 in C. prehensilis seem to be nearly similar, except that in the single 

 young specimen available, the fifth and sixth sternebra had not vet 

 united, and the seventh sternebrum is extremely small, a minute 

 nodule. The cartilaginous tips of the eighth and ninth ribs both adjoin 



