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U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



external characters. Two of these differ only in the presence or absence of hair on the soles 

 and may, perhaps, be best considered as one; the third is similar in skull, but differs in external 

 characters ; the fourth is so distinct as perhaps to be worthy of generic separation. 



The molar teeth appear to be very similar in all, decreasing regularly from anterior to poste 

 rior, each about two-thirds the length of its predecessor in the upper jaw, the disproportion 

 rather less in the lower. The enamel folds are much more superficial than in the South American 

 species, so much so as to render it difficult to detect any particular pattern of indentation. 

 When the crowns wear plane, instead of loops we see usually only a slightly irregular border 

 of enamel encircling the dentine as a single island. The posterior upper molar is nearly circular, 

 and very small ; it has no appreciable indentations whatever ; the middle molar has one median 

 internal, and two rather obscure external, and one at or near each end ; the third has two 

 alternating indentations on each side. In the South American specimen 'referred to, the inden 

 tations, even in the much worn teett, are as deep and as distinct as in Arvicola or Neotoma; those 

 of opposite sides of the same tooth are opposite each other, and almost touch ; there are two on 

 each side the anterior, and one median on each side of the middle and posterior, thus presenting 

 a very different appearance. How constant this may be I cannot say, but Waterhouse's figures 

 of skulls represent the same thing in nearly all the South American species. These differences 

 in the teeth may be tabulated as follows : 



It is very possible, however, that some South American species may be more like ours in the 

 character of the skull, especially the species of Habrothrix. 



The first group embraces species most similar in form to the South American, especially to 

 Calomys. Here the size is very small, none exceeding four inches in the flesh to the root of the 

 tail. The ears are large, with short close hair ; the tail is two-thirds the length of the body or 

 else a little longer. The belly and feet are generally white ; the bright fulvous above of many 

 South American mice is only faintly represented in one or two species, their golden yellow under 

 parts, never. The soles in one division are hairy to the tubercles, which are usually six in 

 number ; in the other division the soles are entirely naked. The tubercles are all large and 

 conical ; the posterior one especially. The claws are all short and weak ; the hinder longest. 

 The hind feet are longer in proportion than the fore ; the hand half the length of the hind foot. 

 The bristly hairs of the back are soft, and only a little longer than the under fur, so that they 

 are but little conspicuous. 



The skull has the superior edge of the orbit rather sharply angular, but not provided with a 

 raised crest or bead, such as is described in the South American species. In the smaller ot 

 these, however, this crest is said to be wanting, as it is in a skull before me from Chile, in which 

 the orbital edge is only sharper than in our species. The molar teeth already described are much 

 the same in all the North American groups, and bear a close relationship (judging from descrip- 



