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184 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 





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being narrow. This style of Mus is siil)stuntially repeated in Hesperomyn, 

 Reithrodon, and Signwdon, though in these the pahitc does not run quite so 

 far bacii, stopping at or just behind the posterior ijorder of the last molars. 

 In Neotoma, on the other hand, the reverse occurs ; for here the palate of 

 Evotomys is almost repeated, in that the excavation runs forward to oppo- 

 site the interspace between the last and pe.u'.timate molars; but there is 

 this difference, that in Neotoma the posterior outline of the palate is deeply 

 concave, and ith sides run back continuously with the pterygoids. 



The under jaw of Evotomys is unmistakably Arvicoline in its sharp, 

 twisted, and upward-bent hamular process, reaching up to tho level of the 

 molar crowns. This form of the descending process is constant, so far as we 

 are aware, and marks the gubfamily Arvicolinte from the MurincB ; for, in these 

 last, the same process is a flattish, oblique, subquadrate plate, never attaining 

 the level of the molars ; and we have never seen an intermediate form. But 

 the jaw of Evotomys has one character not shared by any other Arvicoline 

 that we know of: the coronoid process does not attain the level of the con- 

 dyle. This is owing, we believe, to its absolute shortness, as the length and 

 obliquity of the condylar process itself upjtears about the same as in other 

 Arvicolines. This state of the coronoid is only elsewhere found, among the 

 genera we have studied in the preparation of this memoir, in Ochetodon and 

 Hcsperomys proper ; for in the Onychomys and Oryzomys grou))s of this last, 

 ill Neotoma, Sigitiodon, Mus, and all Arvicolince, the apex of the coronoid 

 mounts as high as, or even surmounts, the condyle. 



The auditory bullae of Evotomys are remarkably large, exceeding in their 

 size and inflation those of any other genus with which we are acquainted. 

 Thus, they are absolutely almost as large as in Arvicola amphibius, an animal 

 twice as big. Tiie nasal bones run back about as far as the nasal branches 

 of the preniaxillaries, both stopping abruptly opposite the anterior roots of 

 the zygoma, and thus considerably in advance of the orbits. It is much the 

 same in Myodes and Synaptomys; in other Arvicolines, in Mus, Hesjiero- 

 tnys, &c., these bones may be of decidedly different lengths, and one or the 

 other — generally the prcmaxillarios — extends to the orbital region of the skull. 

 As in all Arvicolince, the upper incisors are broader than deep; and ns in all 

 these, except Myodes and Synaptomys, the under incisors run past the last 

 molar up the condylar process of the jaw. 



Tlie foregoing appear fo be the tdiief characters of Evotomys, if f,h(!y be 



mm 



