122 MONOGRAMS OF NORTH AMERICAN RODENTIA. 



between the two outer ones. In Ochetodon, the same molar has four roots: 

 three large ones, just as in Ilespcromys ; and an additional very small exterior 

 one, midway between the other two exterior ones. The second molar (in No. 

 2282 for example) has only three evident ones, but a similar, only very minute, 

 fourth one also appears. The last upper molar makes but two perforations. 

 Some difference in the molar crowns of Hesperomys and Ochetodon, correspond- 

 ing to the different rooting in the two genera, may yet be established. This 

 additional root of Ochetodon is an approach toward Mas proper; for, in an 

 example of M. decumanus before us, we find that the alveolus of the upper 

 anterior molar is perforated with six distinct holes. 



There are several remarkable peculiarities in the skull besides the grooved 

 upper incisors. Prominent among these is the singular shape of the descend- 

 ing process of the under jaw. This is really a subquadrate plate, retaining 

 the essential characters that here mark off Murince from Arvicolinee, but, nev- 

 ertheless, when viewed in lateral profile, more nearly resembles the charac- 

 teristic hamular process of the Arvicolince. This appearance is due to the 

 abrupt inward inflection nearly at right angles of the lower border of the 

 process ; nothing like this is seen in Hesperomys, &c. The coronoid is very 

 small and acute, and curves very obliquely backward. The inside of the 

 ramus of the jaw shows, as usual, a ridge denoting the passage of the under 

 incisor up toward the condyle. The skull as a whole is broader and shorter 

 than in Hesperomys; the lengthwise convexity on top is greater, and the cra- 

 nium is quite as wide across the parietal portion as between the zygomata. 

 The latter dip deeply down to the level of the palate, but do not stand out 

 much, their middle portions being nearly parallel. There appears to be a 

 peculiarity in the anteorbital foramen, which is settled far into the zygomatic 

 part of the maxillary, and, of a consequence, appears almost circular above, 

 suddenly contracting into a narrow slit below (somewhat as in Oryzomys; in 

 Hesperomys proper, the broad upper part of the foramen is rather pyriform, 

 and narrows gradually below J. The incisive foramina are large and very long, 

 reaching from a point opposite the anterior molars almost to the incisors. The 

 palate ends behind abruptly as a transverse shelf, opposite the back border of 

 the last molars; but this shelf-edge is very narrow from side to side, owing 

 to the close approximation of the nearly parallel pterygoid ridges; and, on 

 either hand, the palatal level almost directly continues into the shallow 

 depressions, of unusual width, that lie between it and the bullae auditorise. 



