1880.] SPECIES OF KF.ITHRODON. 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 1 '0 



„ head r6o 



,, nose to eye O-S.i 



„ nose to ear-orifice 1 -30 



„ ear-conch 0'G5, 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 extraor- 

 dinary resemblance to Sifjmodon 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 vvhicli 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 shaj)e of 

 the molars is similar, and different from that of the other species of 

 lleithrodon, 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 >Sigmodon by the 

 theory of " mimicry," as the likeness is quite as strong in the 

 cranial as in the external characters, and also because ISiymodon is 

 not as yet recorded from Venezuela — though I believe it probably 

 will be, as I have recently seen what 1 believe to be a specimen of 

 that genus obtained in Ecuador. 



There is of course another explanation possible, namely that 

 R. aJstoni 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 apjjearance 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 import- 

 ant a character, at least in the New-World INIurines, as they have 

 always been taken to be, and would on the whole be strongly 

 confirmatory of Dr. Coues's opinion ' as to the future necessity of 

 amalgamating all the New-^yorld ]Murin8e (except Neotoina) into 

 1 Mon. ISr. Am. Eod. p. 32 (1877). 



