42 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Length from nose to root of tail 



of tail 



from nose to ear 



In. Lines 



3 7 



3 9 



1 



Length of tarsus 



of ear 



. 



In. Lines. 



10 



6 



Habitat, Bahia Blanca, (September.) 



M. 



Musculus, the most evident points of distinction consist in the greater proportionate 

 length of the nasal and frontal bones, and the slenderness of the zygomatic arch 

 in the former animal. Length of skull 11 lines, width 6 lines, distance between 

 front molar and outer side of incisors of upper jaw 3| lines, length of nasal 

 bones 4| lines. 



The dentition is figured in PL 34, figs. 2. b, and 2. c. 



" Whilst bivouacking one night on shore, amongst some sand hillocks, this 

 mouse, with its tail singed, leapt out of a bush which was placed on the fire. Its 

 hind legs appeared long in proportion to the front, and it did not appear to be 

 very active in endeavouring to make its escape." — D. 



Mus 



M. Musculus 



proportion than in the latter, the ears are slightly larger, the tail is longer, and 

 so are the tarsi. The large ears, long tail, and comparatively large size of the 

 feet, combined with the greater size of the animal itself, will render it easy to 



M. 



From the last 



mentioned animal it moreover differs in having the head larger in proportion, 

 the fur longer, and the colouring of the upper parts of the body somewhat darker. 

 The white fur is almost confined to the under parts of the body, and there is but 

 a small tuft of white hairs behind the ears, whereas in M. bimaculatus, the white 

 fur extends considerably on the sides of the body, the outer side of the limbs are 

 white, and there is a large and conspicuous white spot behind each ear. 



In M. elegans the whole sole of the tarsus and the carpal tubercles are 



covered with hair. 



Mus bimaculatus the hinder half 



covered with hair, and in M. gracilipes both the hinder half is covered, and there 

 are some scattered hairs extending almost to the two tubercles, which are situated 

 at the base of the longer toes. 



The genus Eligmoclontia of M. F. Cuvier, founded upon a species of mouse 

 from Buenos Ayres, possesses nearly the same characters as the subgenus 

 Calomys, established by me in the Proceedings of the Zoological Society for 

 February 1837, and which included the animal above described, and two other 

 species {31. bimaculatus and M. gracilipes). M. Cuvier's genus is distinguished 

 by there being only one large tubercle on the under side of the tarsus, and in 

 having the carpal pad covered with hair as well as the pad of the tarsus. In 





