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118 



MONOGRAPHS OF NORTU AMERICAN RODENTIA. 



NoTK.— Ah prcliniinnrj to considoralinn of thu gcnns Ochetoion, a notice or tho gontb American 



Iti'ilhwilon 18 introduced, tu facilitutu cuuiimriwu butweeii tlio two. 



Genus RBITDRODON, Waterhouse. 



Heillirodov, WATEnilouSK, Proo. Zool. Soc. 1837, 89 ; Zool. Voy. Bcnglv, pt. ii, 183U (mm LcCoutr, Boird). 



Diagnosis.— Upper incisors snlcato. liostral portion of tbo skuiriurgti in proportion to the cra- 

 uiul, producing u bigh convex forubcod; zygomatic widtb of skull about twu-tbirds its length ; iuter- 

 urbitnl portion narrow ; poalerior nnrcs contracted from close approximation of tbo pterygoids; palate 

 with lateral paired fussB), and ending nearly apposite or Iwyond the last niolor ; incisive forauiiua very 

 lung, extending to or beyond tbo lirst molar ; coronoid process small aud cxUected ; condyloid narrow and 

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

 deep. Size largo ; form stout and compact ; bead broad and arebed ; eyes large and prominent. Kurs 

 uiuileratu, pilous. 8ules liuiry behind ; lateral toes subcqual and extremely short. Tail short, about oue- 

 bulf tbu head and body, moderately hairy. 



Notwithstanding 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 Waterhouso originally 

 pointed out, and agrees with MuriiitB in every esseatial 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 M»rina in the North American genus Ochelo- 

 don. liut we are satislied, from our study of Waterbouse's perspicacious accounts, that in this case the 

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

 several points of cranial structure, and many external features, Oohetodon is very unlike UtilhrodoH. The 

 former includes little animalrwboso general appearance may be summed up as that of Mm mvscHlut, 

 while the latter comprises largo species with a strikingly rabbit-like ai>pearaace (see also under 

 Uchetodon). 



Indeed, wo arc not satisfied that tbo three species originally referred to Heilhrodon do not themselves 

 share sulcatioH of the superior incisors as a character only incidental to two diifureat types of at least 

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

 tures. There is a remarkable difi'erence in the shape of the plate inclosing the onteorhital foramen, in 

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

 ters in the proportions of tbo 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 

 Oriizmiya from each other. We may t^tbulato the distinctive characters of Keitkvodon, and of Euneomy», 

 as we have termed the now subgenus), as follows : — 



Ileillirodoii, Watekiiuuse. — Anterior root of zygoma deeply cmargiuated in front. Palate ending 

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

 excavations. Pterygoid fossiB deeply excavated, aud tbo 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 obliqne. — Type, H, cmiouhida. 



ICuncomije, KoB., 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 fosso) shal- 

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

 process of lower Jaw (lat internally ; descending process angular; coronoid process nearly vertical. — 

 Typk, a*, fhinchithidcs. 



There are other differences in the relative proportions of the sknll ; and doubtless additional diag- 

 nostic features could bo adduced, but these will suffice. 



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

 doiit pattern, calling for little further comment. The incisors, as usual in tbo subfamily, are deeper antero- 

 puKtoriorly than they are wide across. The molars decrease regularly iu size and complexity from before 

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

 salieiicii's on each side being about opposite each other, while the two rcentrances from each side, which 

 meet on tbo face of thu 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 ol' tlin otliers. The front under molar shows on the onler side three perfect reentrant loops, nnd mi 

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

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

 Bometiuics, however, with trace of another. Thu buck lower molar has but one fold on either side; these 





