118 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



Niui:.— As preliminary to consideration of the genus Ochelo&on, a notice of the South American 

 Seilhrodoii is introduced, to facilitate comparison between the two. 



Genus RBITHRODON, Waterhousa 



Rdthrodon, Waii urioi sk, Proc. /.mil. Sue. 1837,29; Zool. Voy. Beagle, pt. ii, 1839 (nee LeConte, Baird). 



Diagnosis. — Upper incisors snicate. Rostral portion of the skull large iu proportion to the cra- 

 nial, producing a high convex forehead; zygomatic width of skull about two-thirds its length ; inter- 

 orbital portion narrow ; posterior Dares contracted from close approximation of the pterygoids; palate 

 with lateral paired fossa;, and ending nearly opposite or beyond the last molar; incisive foramina very 

 long, extending to or beyond the first molar ; coronoid process small and exllected ; condyloid narrow and 

 very oblique; descending process large, Bubquadrate, the emargination between this and the condyle 

 deep. Size large ; form stout and compact ; head broad and arched; eyes large and prominent. Ears 

 moderate, pilous. Soles hairy behind ; lateral toessubequal aud extremely short. Tail short, about one- 

 half the head aud body, moderately hairy. 



Not withstanding this genus has certain peculiarities of external form that combine to produce an 

 aspect unusual among Murines, it is certainly a true member of the subfamily, as AVaterhouse originally 

 pointed out, aud agrees with Murines iu every essential feature. It may be recognized at once by the 

 longitudinal grooving of the upper incisors, a feature not shared by any other South American mice as 

 far as is known, and only again met with among American Murinw in the North Americau genus Ocheto- 

 don. But we are satisfied, from our study of Waterhouse's perspicacious accounts, that in this case the 

 grooving of the incisors is a character merely incidental to two genera otherwise widely separated ; for in 

 several points of cranial structure, and many external features, Oclielodon is very uulike Reithrodon. The 

 former includes little animals whose general appearance may be summed up as that of Mits mnscutiis, 

 while the latter comprises large species with a strikingly rabbit-like appearance (see also under 

 Uchctodon). 



Indeed, wo are not satisfied that the three species originally referred to Beithrodon do not themselves 

 share sulcation of the superior incisors as a character only incidental to two different types of at least 

 subgencric value. They certainly differ notably in several cranial peculiarities as well as external fea- 

 tures. There is a remarkable difference in the shape of the plate inclosing the anteorbital foramen, in 

 the details of the palatal structure, aud in the form of the under jaw ; while there are external charac- 

 ters in the proportions of the tail, feet, aud ears. Iu the respects of palatal structure aud the form of 

 the anterior zygomatic root, there is a curious analogy to the same points as distinguishing Sigmodon and 

 Oryzumya from each other. We may tabulate the distinctive characters of Reithrodon, and of Ennmmya, 

 as we have termed the new subgenus, as follows: — 



Reithrodon, Watekhouse. — Anterior root of zygoma deeply emarginated in front. Palate ending 

 much behind the molar series, and showing a median ridge intervening between lateral paired deep 

 excavatious. Pterygoid fossa? deeply excavated, and the bones very closely approximated. Incis- 

 ive foramina reaching beyond first molars. Condyloid process of lower jaw concave internally; 

 descending process rounded off inferiorly ; coronoid process very oblique. — TYPE, R. cuviculoides. 



Euneomye, Nob., Proc. Acad. Thila. 1374, lrto. — Anterior root of zygoma about straight iu front. 

 Palate ending nearly opposite the last molais, slightly ridged or excavated. Pterygoid fossai shal- 

 low and these bones less approximated. Incisive foramina reaching only to first molars. Condyle. d 

 processof lower jaw flat internally; descending process angular; coronoid process nearly vertical. — 

 Type, /.'. ehinckilloides. 



There are other differences in the relative proportions of the skull ; and doubtless additioual diag- 

 nostic features could be adduced, but these will suffice. 



Aside from the grooving of the upper incisors, the dentition of this genus is of the ordinary sigmo- 

 don t pattern, calling for little further comment. The incisors, as usual in the subfamily, are deeper antero- 

 posterior!}' than they are wide across. The molars decrease regularly iu sizo and complexity from before 

 backward; the front upper one has two external and two internal folds of enamel, the resulting three 

 saliencies ou each side beiug about opposite each other, while the two reen trances from each side, which 

 mr.t on the face of the tooth, are alternate. The other two upper teeth have but one internal aud two 

 external folds ; the posterior one of the latter, in each tooth, being much deeper and more tortuous than 

 any of the others. The front under molar shows on the outer side three perfect reentrant loops, and ou 

 the inner side two such, but may have one additional slight loop on each side, from indentations of the 

 anterior end of the tooth. The mid-lower molar has two folds on the outside and oue on the inner side, 

 sometimes, however, wnu trace of another. The back lower molar has but oue fold ou either side; these 



