MURIDA—SIGMODONTES—OCHETODON MEXICANUS. 129 
We have no doubt that Professor Sumichrast’s specimens represent ty pi- 
cally the Reithrodon mexicanus of De Saussure. ‘Though they present some 
discrepancies in dimensions, De Saussure’s measurements, he says, are prob- 
ably a little under the mark, while Sumichrast’s specimens seem a little 
stretched ; this is enough to bring about perfect concordance. 
Our Louisiana specimens are highly interesting, as showing for the first 
time the occurrence of the true Mexican form in the United States. They 
are unquestionably identical with Sumichrast’s examples from Tehuacan, and 
agree even better than these with De Saussure’s figures. The discovery of 
this style of Ochetodon in the United States is especially important in its bear- 
ing upon the identification of Mus carolinensis, Aud. & Bach. Nos. 7748-7749 
agree with the account of carolinensis in length of tail, and in the dullness of 
color of the under parts; and it may be, after all, that this long-tailed meai- 
canus ranges coastwise up to the Carolinas. Still, there are discrepancies 
that cannot be overlooked between Audubon’s description and the present 
animal, especially as to the size of the feet; and we are not at present war- 
ranted in calling the form Ochetodon carolinensis. The latter name must be 
assigned, with a query, as a synonym of humilis, at least until we find the 
long-tailed large-footed form in Carolina. 
That there is a regular gradation in length of tail and size of foot 
between humilis on the one hand and mezicana on the other is undeniable ; 
and if we could have proven in the genus Ochetodon the same amount of varia- 
tion with locality that has always been admitted in the case of Zapus hud- 
sonius, and that obtains with Hesperomys leucopus and Arvicola ripurius, we 
should be forced to merge the three supposed species into one, with two 
geographical variations. Failing in this, however, at present, we can, at any 
rate, conveniently mark off three kinds of Ochetodon. The following analysis 
(in which coloration, which though an aid in identification, may not be always 
reliable, is omitted) will, it is believed, enable us to readily distinguish ninety 
per cent. or more of our specimens :— 
A. Tail shorter than head and body (at most barely equaling head 
and body). 
a. Hind feet under 0.70 long (usually 0.55-0.65) - - - - - - - HUMILIS. 
B. Tail longer than head and body. 
b. Hind feet under 0.70 (exceptionally just reaching 0.70)- LONGICAUDA. 
c. Hind feet over 0.70 (rarely, if ever, falling to 0.70)--.. MEXICANUS. 
9M 
