MLJKID.E— SIGMODONTES— SIGMODON. 



33 



whereas they do not appear to afford grounds for any such subordination. 

 Thus, Onychomys seems to ns fully as different from Hesperomys proper as 

 Sigmodon is ; and we should judge the same to be the case with such forms 

 as Abrothrix and Oxymicterus. But as we are not now undertaking a critical 

 revision of the whole sigmodont tribe, we prefer to accept the groups as we 

 find them, provided they are established with any considerable show of reason, 

 rather than present decided innovations; and for the present, in order to 

 bring out the groups in the strongest relief, we accord generic rank to some 

 that probably we should reduce to subgenera upon final survey of the whole. 



• The principal diagnostic points of the skull of Sigmodon are those men- 

 tioned in a preceding paragraph. The shortness of the skull, in comparison 

 with the great width, resulting from the widely divergent zygomata ; the short, 

 swollen, rostral portion ; the arched frontal profile ; the curious little pointed 

 process of the lamellar plate of the maxillary that bounds the foramen 

 (wanting in other groups we have examined — even in Oryzomys) ; the posi- 

 tion and peculiar sculpture of the palatal shelf; the small bulla? osseas; the 

 bead on the orbital edges, — all are strong characters, in peculiar combination. 

 We might easily describe several additional, more minute features, but this 

 seems unnecessary. The following table shows the size, proportions, and 

 variations of the skull of the single species of the genus we have examined. 



Table VI. — Measurements of fourteen skulls of species of Sigmodon. 



Dentition. — Selecting an average adult example, as No. 4763, in which 

 the teeth are full-grown, yet not, so worn as to deface the pattern, we find as 



3 M 



