MAMMALIA. 
75 
in M. bimaculatus at any age. As soon as the molar teeth are worn, the folds of 
enamel penetrate the body of the tooth on each side, and those of one side alter- 
nate with those of the other, — in fact, they very nearly resemble those of the 
Hamsters ( Cricetus ). 
I have selected the molar teeth of Mus Rattus and M. bimaculatus for com- 
parison, since I happened to possess specimens displaying both the young and 
adult states of each. But had I selected, on the one hand, almost any of the 
species brought from South America by Mr. Darwin, and, on the other hand, 
the Mus musculus or M. decumanus, I should have had to point out the same 
distinctions — the former agreeing in dentition with M. bimaculatus , and the latter 
with M. Rattus. 
The differences pointed out, between the molar teeth of Mus Rattus and 
those of M. bimaculatus , I cannot but consider as important, since all the Old 
World species of Mus which I have yet had an opportunity of examining (and 
they are numerous) agree essentially with the former, whilst the only Mus from 
S. America (excepting M. Musculus and M. decumanus, which are carried in ships 
to all parts of the world) in which I have as yet found molar teeth like those of 
M. Rattus, is the Mus Maurus , and this it has been stated is possibly a variety 
of M. decumanus. 
Although as yet I have not met with species in the Old World possessing 
the characters of the South American Muridce, among those of North America, 
several have come under my observation. The Mus leucopus, Symidon hispidum, 
and the species of Neotoma certainly belong to the same group,* as does also the 
species of the Galapago Islands, described in this work under the name Gala- 
pagoensis. 
These considerations have induced me to separate the South American mice 
from those of the Old World, — or rather from that group of which M. decumanus 
may be regarded as the type, — and to place them, together with such North 
American species as agree with them in dentition, in a new genus bearing the 
name Hesperomys.\ 
Whether this group be confined to the Western hemisphere or not, I will not 
venture to say, but I think I may safely affirm that that portion of the globe is 
their chief metropolis. 
The species of the genus Hesperomys, which depart most from the type — whose 
dentition is least like figs. 5, a, and 5, b, Plate 33. or 6, a, and 6 , b, of the 
* I am acquainted with seven North American Species of Muridce , all of which possess the dentition of Hes- 
peromys. 
+ ’E(T7repos, West, and Mus. 
