282 Mr. 0. Tliomas on 



liighly specialized, by tlie great development of its claws, for 

 a fossorial life. 



Moreover, its external resemblance to A. macroni/x, in- 

 habiting the same districts, amounts practically to identity, 

 there being absolutely no single character, of size, proportions, 

 or colour, which would make the keenest-eyed " splitter " 

 suppose that the skin of R. fossor did not belong to 

 AJcodon, though in the skull the difference is complete. We 

 have therefore here the striking phenomenon of a large genus 

 like Alcodon with a few specialized fossorial species, and 

 another smaller genus, with its similarly fossorial species 

 exactly mimicking the corresponding species of the larger 

 group. No doubt the resemblance is not true mimicry in the 

 technical sense, being presumably due to identity of life and 

 local conditions, but it is very striking nevertheless. 



In this connexion attention may be called to the resem- 

 blance of Reithrodon Ahtoni to Sigmodon^ mentioned in the 

 original description of the former *, a resemblance so close as 

 again almost to amount to identity so far as external appear- 

 ance is concerned. 



Echimys decumanus^ sp. n. 



Size medium. Spines practically confined to the dorsal 

 surface, the fur on the sides being only hispid, not spinous ; 

 strongest spines of back about 20-25 millim. in length and 

 O'C-0'8 millim. in breadth ; greyish white basally, black 

 terminally. General colour above coarsely grizzled sandy 

 fawn, not unlike that of bright pale-coloured specimens of 

 Mus decumanus, greyer anteriorly, more fawn-coloured poste- 

 riorly. Spinous area along back rather darker, owing to the 

 black tips of the spines, sides paler and greyer. Face 

 grizzled grey, with less tinge of fawn than the back j patch 

 between eye and ear brownish grey. Underside and inner 

 sides of forearms and hips pure white, the hairs white to their 

 bases ; upper surface of hands and feet also white or with a 

 slight wash of greyish fawn. Tail uniformly thinly haired, 

 the hairs not hiding the scales, not tufted or pencilled, black 

 above and white below. 



Nasals surpassing premaxillse by about one sixth of their 

 length ; supraorbital edges less heavily ridged than in some 

 of the allied species ; posterior extension of ridges interrupted 

 on parietals ; sides of anterior palatine foramina markedly 

 convex outwards ; pterygoids much twisted, narrower and 



* P. Z. S. 1880, p. 693. 



