220 
in. lin. 
Length of the head and body, about ...... 6 6 
OfQUMeseHNl, 2.5) 6 aes, ore sratieovereuxtuamipae 9 0 
GEALREMICAN cone che icice choise oteihys oa 1r"9 
——————— Of PHEIERTS <2 eo: . 2c sce ccgoe oe se 0 8 
of the fore foot and claws ........ 0O 9 
Gite POVESATMN cc cies chins oes cs. Wile ol 
AFIMG MAIS ee ss Piva aiavs vce oie hws 1 6 
of the hind foot and claws........ 1-4 
Obs.—The species whieh are here described under the names of 
HI. latimanus and H. bicolor do not fall with facility under either of 
the subgenera proposed by Mr. Waterhouse; and neither do they 
agree with the species which are brought by Wagner and Burmeister 
under the generic or sub-generic name of Holochilus. They consti- 
tute rather a group of themselves, which I will here briefly charac- 
terize. 
But I may premise, before doing this, that it seems to me needless 
to encumber science with another name ; for lam scarcely of opinion 
that this or any other of the groups into which the genus Hesperomys 
has been divided, should be regarded as more than divisions for 
the convenience of description and identification. A group which is 
characterized in as purely superficial a manner as are those now under 
review, should, to hold a recognizable place in any system, have a 
well-defined outline: although removed to but a little distance from 
allied groups, the intervening space should be quite clear of outliers 
from either side. There are perhaps but few such groups to be met 
with, but there are some. It is probable that such occur in the Sori- 
cide, and amongst the Bats I can cite two good instances. The genus 
Nycticejus of Asia and Africa differs from the heavy-built Vesper- 
tiliones (Scotophilus) in a trifling but constant manner, the charac- 
teristic differences appearing to be but feeble in a generic signification ; 
but immensely strengthened by their constancy. The genus contains 
several well-marked species, all of which possess the same charac- 
teristics in a nearly equal degree. Another and equally good instance 
is the genus Lasiurus, confined to the New World. 
I have in vain sought for anything like this amongst the subgenera 
into which Hesperomys has been divided ; I even find sufficient va- 
riation in different individuals of some of the species to endanger 
these divisions. For instance, the difference in the length of the 
tail in adult specimens of H. longicaudatus is very considerable, and 
the ears in H. elegans vary in size in a remarkable manner, so much 
so, as to give the idea of two distinct species. But the peculiarity 
is wholly superficial, and is highly variable. By these variations the 
subgenera Calomys and Phyllotis are, as it were, mixed up and 
blended, and their value impaired. The genus itself—Hesperomys 
—may more properly be likened to the genera of Vespertilionide of 
which I have spoken, as it is distinct from the cosmopolitan genus 
Mus in one only, but very constant pomt of dissimilarity — the 
presence of a rather greater number of folds of enamel in the crowns 
of the molar teeth. We do not know the exact degree of importance 
