32 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



tened nail. Hind feel very long, generally aboul equaling distance From nose 

 to car; 1st and 5th toes subequal and very short, the latter reaching lint little 

 beyond tin' basal join! of the 4th. Sides entirety naked, granular at bases 

 of toes, perfectly smooth behind, 6-tuberculate, the tubercles all small, the 

 hindermosl nol lengthened and linear as in Mus; the 2d tubercle just outside 

 and a little in advance id' this one; a tubercle just inside the base of the 1st 

 and fit h 1o. s respectively ; one between bases of 2d and 3d toes ; one bet ween 

 liases of 3d anil 4th toes. 



Although this form of sigmodonl Murines is undoubtedly strongly marked, 



yel we eannot see that it stands apart from the rest so tar 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, it 

 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, JVeotoma, 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 Onjzomys, 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 Sigmodon 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 sufficiently familiar with all the exotic American Murince to come to a 

 final conclusion : but we suspect that it will in time be found advisable or 

 necessary to combine must 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 superspecitie 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; 



