490 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
B. arizonae is about the size of the living Lligmodontia morgani of 
South America, which it resembles in many respects, and at the same 
time differs from Peromyscus, notably in the anterior extension of the 
masseteric crest into a prominent knoblike process anteroexternal to 
the anterior root of the first cheek tooth, the dorsal position of the 
mental foramen, the reduction of the last lower cheek tooth, and the 
depth of the sulcus between the capsular and coronoid processes. B. 
arizonae differs from #. morgani in having a deeper lower jaw, more 
brachydont teeth, a more procumbent incisor, a distinctly better de- 
veloped notch in the anterior lobe of the first cheek tooth, slightly 
less reduced third cheek tooth, and somewhat more dorsally placed 
mental foramen which is closer to the prominence at the anterior end 
of the masseteric crest. The shortness of the symphysis indicated by 
Gidley cannot be certainly determined, as none of the jaws are suffi- 
ciently complete anteriorly, and the incisor appears to have slid back- 
ward to a different position in each. 
ONYCHOMYS BENSONI Gidley 
The right ramus of a mandible, No. 10509, with all the teeth, but 
lacking part of the coronoid and angle, is the type and only specimen 
of the Benson Onychomys. As noted in the above description of the 
Bensonomys arizonae material, Gidley’s figure of the type of Ony- 
chomys bensoni is labelled Eligmodontia arizonae, so that O. bensoni 
is actually shown in his plate 34, figure 15, rather than plate 35, figure 3. 
Onychomys bensoni is rather close in size to Onychomys torridus. 
It exhibits the same cuspate type of tooth structure seen in modern 
Onychomys, with the narrow, single cusped anterior loph of the first 
lower cheek tooth, but the teeth are a little lower crowned and the 
cusps perhaps a little more conical. The third lower cheek tooth is 
distinctly less reduced, exhibiting a low but much less abbreviated 
talonid portion than in OQ. torridus or even O. leucogaster. 'The size 
and height of the coronoid indicated by Gidley cannot be determined, 
but the basal portion appears to have been of greater anteroposterior 
extent, as indicated by the length of the broken edge. 
SIGMODON MEDIUS Gidley 
Five lower jaws, four of which exhibit all the cheek teeth, represent 
a species of Sigmodon in the Benson fauna. The type, No. 10519, a 
right ramus, which includes the incisor as well as the cheek teeth, 
and an associated fragment of the right maxilla, with the first two 
cheek teeth, believed to be from the same individual. 
Sigmodon medius is smaller than the modern forms Sigmodon his- 
pidus and Sigmodon sanctae martae with which Gidley made compari- 
sons. It is intermediate in size between the two Curtis ranch species, 
