32 MONOGRAPHS OF NORTH AMERICAN RODENTIA, 
tened nail. Hind feet very long, generally about equaling distance from nose 
to ear; 1st and 5th toes subequal and very short, the latter reaching but little 
beyond the basal joint of the 4th. Soles entirely naked, granular at bases 
of toes, perfectly smooth behind, 6-tuberculate, the tubercles all small, the 
hindermost not lengthened and linear as in A/us; the 2d tubercle just outside 
and a little in advance of this one; a tubercle just inside the base of the Ist 
and 5th toes respectively ; one between bases of 2d and 3d toes; one between 
bases of 3d and 4th toes. 
Although this form of sigmodont Murine is undoubtedly strongly marked, 
yet we cannot see that it stands apart from the rest so far as it is tacitly sup- 
posed to. A good deal that has been written about its peculiarities of denti- 
tion might be advantageously toned down; in fact, we do not find much, if 
any, greater dental characteristics than those slightly superspecific ones mark- 
ing several other forms usually ranked as subgenera of Hesperomys. The 
loops of enamel on the posterior molars do form a sort of sigma, but it is 
usually a broken and always a distorted one, never more evident than in some 
other sigmodont forms. The pattern of the teeth is fully as changeable with 
age as it is in Hesperomys, Neotoma, Mus, and other genera; and it is only to 
a particular stage of the crowns that the details of pattern, usually ascribed to 
the genus, hold good. Moreover, we have, in the section Oryzomys, a perfect 
link between Sigmodon and the ordinary small Hesperomys of America. The 
connection is so close and complete, that, in fact, we should almost think Ory- 
zomys ought to take place as a subgenus of Sigmodon rather than of Hespero- 
mys; or, if retained where it is now, Sigmodon ought to be laid over against 
it as another subgenus of Hesperomys. In external characters, Oryzomys 
agrees better with Stgmodon than it does with ordinary Hesperomys ; the two 
are so much alike, in fact, that the relative length of the toes and the com- 
parative size of the ears are the most readily-expressed differences. We are 
not suthciently familiar with all the exotic American Murine to come to a 
final conclusion: but we suspect that it will in time be found advisable or 
necessary to combine most of the species of the sigmodont Mures into one 
genus (for which the name Sigmodon, antedating Hesperomys, would have to 
be adopted), with several subgenera or groups of species; for, with the 
exception of Neotoma, perhaps Holochilus, and possibly one or two others, 
the various superspecific groups seem to differ from each other by characters 
of about equal or equivalent value. The impropriety is, that it is at present 
customary to hold some of these groups for genera, others only for subgenera ; 
