134 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
being narrow. This style of Mus is substantially repeated in Hesperomys, 
Reithrodon, and Sigmodon, though in these the palate does not run quite so 
far back, stopping at or just behind the posterior border 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 penultimate molars; but there is 
this difference, that in Meotoma the posterior outline of the palate is deeply 
concave, and its 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 the level of the 
molar crowns. This form of the descending process is constant, so far as we 
are aware, and marks the subfamily Arvicoline trom the Murine ; 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 appears 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 
Hesperomys proper; for in the Onychomys and Oryzomys groups of this last, 
in Meotoma, Sigmodon, Mus, and all Arvicoline, the apex of the coronoid 
mounts as high as, or even surmounts, the condyle. 
The auditory bulla 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. The nasal bones run back about as far as the nasal branches 
of the premaxillaries, 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 Arvicoline, in Mus, Hespero- 
mys, &e., these bones may be of decidedly different lengths, and one or the 
other—generally the premaxillaries—extends to the orbital region of the skull. 
As in all Arvicoline, the upper incisors are broader than deep; and as in all 
these, except Myodes and Synaptomys, the under incisors run past the last 
molar up the condylar process of the jaw. 
The foregoing appear.to be the chief characters of Hvotomys, if they be 
