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Show 1880.] SPECIES OF REITHRODON. 693 Measurements of the type, an adult male in spirit:- in. Length of head and body 5*3 tail 3*6 ,, hind foot without claws 10 ,, head 1*65 „ nose to eye 0*65 ,, nose to ear-orifice 1 *30 „ ear-conch 0*65, breadth 0*62 „ forearm and hand 1*28 „ tibia from knee to sole 1*3 „ skull 1-36 One of the most interesting facts about this species is its extraordinary resemblance to Sigmodon hispidus, referred to above. So strong is this resemblance that I can fairly say that, with the exception of the grooved incisors, there is not one single character, cranial or external, which would make me hesitate to refer this specimen to the genus Sigmodon ; while, as regards the species, the only characters of any sort on which it might be separated from S. hispidus are the decidedly shorter tarsus and the more distinct concavity of the frontal bone mentioned above. The very shape of the molars is similar, and different from that of the other species of Reithrodon, though, as stated above, the pattern is too worn in the type to be distinguishable; yet the incisors are deeply and distinctly grooved, while in Sigmodon they are especially smooth and convex in front and not in the least showing an approximation to a groove, as is done in some other murine non-grooved forms. It is true that in the other species of Reithrodon the grooves are often shallow and indistinct; but in these species there is no resemblance whatever to Sigmodon, and their locality, the extreme south of S. America, is the very opposite of intermediate. Neither can we explain this resemblance of R. alstoni to Siymodon by the theory of " mimicry," as the likeness is quite as strong in the cranial as in the external characters, and also because Sigmodon is not as yet recorded from Venezuela-though I believe it probably will be," as I have recently seen what I believe to be a specimen of that genus obtained in Ecuador. There is of course another explanation possible, namely that R. alstoni has been independently developed from some Sigmodont form and has nothing to do with Reithrodon, except what is shared by the other American Murinae. Considering how different from the other Reithrodons the general appearance and shape of skull of R. alstoni are, it seems just possible that this may be the case ; and if so, it would show that grooved incisors are not nearly so important a character, at least in the New-World Murines, as they have always been taken to be, and would on the whole be strongly confirmatory of Dr. Coues's opinion l as to the future necessity of amalgamating all the New-World Murinae (except Neotoma) into ° 1 Mon. N. Am. Eod. p. 32 (1877). |