1 94 Bulletin American Museum of Natural History. [Vol. XIX, 

 NOTE ON THE GENUS Euneomys COUES. 



Waterhouse, in founding the genus Reithrodon (P. Z. S., 

 1837, p. 29), included in it two species, R. typicus and R. 

 cuniculoides, which appear to be strictly congeneric. In the 

 'Zoology of the Voyage of the Beagle' (Mammalia, Part n, 

 1839, p. 72), he added as a third species, R. chinchilloides , and 

 gave figures of R. cuniculoides and R. chinchilloides, including 

 the external characters and the skull and teeth of each, and 

 the lower molar teeth of R. typicus. In 1874, Dr. Coues (Proc. 

 Acad. Nat. Sci. Phila., 1874, p. 185), and later in 'Mono- 

 graphs of North American Rodent ia' (1877, pp. 118, 119), 

 from a study of Water house's figures, divided the genus 

 Reithrodon into two groups, to which he gave the rank of sub- 

 genera, making .R. cuniculoides the type of the restricted 

 group Reithrodon, and R. chinchilloides the type and only 

 species of his " subgenus " Euneomys, giving very clearly some 

 of the principal differential characters of the two groups. The 

 more important of these are: (i) "Anterior root of zygoma 

 deeply emarginated in front", in Reithrodon and "about 

 straight in front" in Euneomys; (2) "palate ending much 

 behind the molar series, and showing a median ridge interven- 

 ing between lateral paired deep excavations" in Reithrodon, 

 and "palate ending nearly opposite the last molars, slightly 

 ridged or excavated" in Euneomys; (3) "pterygoid fossae deeply 

 excavated, and the bones very closely approximated" in Re- 

 ithrodon, and " pterygoid fossae shallow and these bones less ap- 

 proximate " in Euneomys; (4) " condyloid process of lower jaw 

 concave internally" in Reithrodon, and "condyloid process of 

 the lower jaw flat internally" in Euneomys; (5) " coronoid pro- 

 cess slender, very oblique" inReithrodon, and "coronoid process 

 very broad, nearly vertical" in Euneomys. To these maybe 

 added (6) the very different enamel pattern of the molar teeth 

 in the two groups, in Reithrodon the folds being transverse with 

 the outer and inner loops alternating, and in Euneomys 

 oblique, with one less fold in each of the last two upper teeth, 

 and in the first two lower teeth, a very radical difference 

 in tooth structure, which alone warrants the generic separa- 



