40 
ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
The most conspicuous characters of the present species consist in the im- 
mense length of the tail, and the great size of the hinder feet. * It is about equal 
in size to Mus Musculus ; its form, however, is somewhat stouter; in colour it is 
much paler and brighter. The head is larger in proportion ; the ears are smaller, 
and more densely clothed with hair ; the fore feet are rather larger, and the fleshy 
tubercle on the under side of the wrist is also larger. The thumb nail is flattened, 
and rounded at the tip, as in Mus Musculus, but is longer, and more distinct than 
in that animal. 
The skull of M. longicaudatus, (Plate 34, Fig. ],) is considerably larger than 
that of the common mouse, but in form scarcely differs from it; its upper surface is 
rather more convex, and the interparietal bone proportionately less. The length of 
the skull is 1 inch ; breadth, 6^ lines ; distance between the fore part of the incisor, 
and the first molar of the upper jaw, 3^ lines. The dentition is figured in Plate 34, 
Figs. 1. b and 1 . c. 
The above account is drawn up from the same specimen as that from which 
Mr. Bennett took his description, and which was brought from Chile by Mr. Cum- 
ing, who states that the animal in question lives in trees, and constructs its nest 
with grass. 
In Mr. Darwin’s collection, I find an animal which agrees in all the more im- 
portant characters with the one above described, but differs in being of a deeper 
colour, (approaching more nearly, in this respect, to the common mouse,) and in 
having the tail a trifle shorter. The skull is about § of a line shorter, but its 
proportions agree precisely : the proportions of the feet, and the general form of the 
animal, also agree. This specimen is likewise from Chile, (Lat 37° 40',) and, accord- 
ing to Mr. Darwin, “ overran the wooded country south of Concepcion, in swarms 
of infinite numbers. Captain FitzRoy, on his return from visiting the wreck of 
H. M. S. Challenger, had the kindness to bring me this specimen. So destruc- 
tive was this little animal, that it even gnawed through the paper of the cartridges 
belonging to the people who were wrecked.” — D. 
* A long tarsus is generally accompanied by a proportionately long tail. I presume that those Mice 
which have long tarsi are in the habit of making great leaps, and that in these leaps, the tail serves to steady 
and balance the body. 
