118 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
Norr.—As preliminary to consideration of the genus Ochelodon, a notice of the South American 
Reithrodon is introduced, to facilitate comparison between the two. 
GENUS REITHRODON, Waterhouse. 
Reithrodon, WATERHOUSE, Proc. Zool. Soc. 1837, 29; Zool. Voy. Beagle, pt. ii, 1839 (nee LeConte, Baird). 
D1aGnosis.— Upper incisors suleate. Rostral portion of the skull large in proportion to the era- 
nial, producing a high convex forehead; zygomatic width of skull about two-thirds its length ; inter- 
orbital portion narrow ; posterior nares contracted from close approximation of the pterygoids; palate 
with lateral paired fossx, and ending nearly opposite or beyond the last molar; incisive foramina very 
long, extending to or beyond the first molar ; coronoid process small and exflected ; condyloid narrow and 
very oblique; descending process large, subquadrate, 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 bebind; lateral toes subequal and extremely short. Tail short, about one- 
half the head and body, moderately hairy. 
Notwithstanding this genus has certain peculiarities of external form that combine to produce an 
aspect unusual among Marines, it is certainly a true member of the subfamily, as Waterhouse originally 
pointed out, and agrees with Murinw in 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 Murine in the North American genus Ochelo- 
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, Ochetodon is very unlike [eithrodon. The 
former includes little animals whose general appearance may be summed up as that of Mus musculus, 
while the latter comprises large species with a strikingly rabbit-like appearance (see also under 
Ochetodon). 
Indeed, we are not satisfied that the three species originally referred to Reithrodon do not themselves 
share sulcation of the superior incisors as a character only incidental to two different types of at least 
subgeneric 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, and in the form of the under jaw; while there are external charac- 
ters in the proportions of the tail, feet, and ears. In the respects of palatal structure and the form of 
the anterior zygomatic root, there is a curious analogy to the same points as distinguishing Sigmodon and 
Oryzomys from each other. We may tabulate the distinctive characters of Ieithrodon, and of Euneonys, 
as we have termed the new subgenus, as follows :— 
Reithrodon, WATERHOUSE.—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 
excavations. Pterygoid fossw 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 yery obliqne.—TyPe, R. curnieuloides. 
Euncomys, Nos., Proc, Acad. Phila. 1874, 185.—Anterior root of zygoma about straight in front. 
Palate ending nearly opposite the last molars, slightly ridged or excavated. Pterygoid fosse shal- 
low and these bones less approximated. Incisive foramina reaching only to first molars. Condyloid 
process of lower jaw flat internally; descending process angular; coronoid process nearly yertical.— 
Type, L?. chinchilloides. 
There are other differences in the relative proportions of the skull; and doubtless additional 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- 
dont pattern, calling for little further comment. The incisors, as usual in the subfamily, are deeper antero- 
posteriorly than they are wide across. The molars decrease regularly in size and complexity from before 
backward; the front upper one has two external and two internal folds of enamel, the resulting three 
saliencies on each side being about opposite each other, while the two reéntrances from each side, which 
meet on the face of the tooth, are alternate. The other two upper teeth have but one internal and 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 reéntrant loops, and on 
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 oh the outside and one on the inner side, 
sometimes, howeyer, with trace of another. The back lower molar has but one fold on either side; these 
