FOSSIL VERTEBRATES FROM SAN PEDRO VALLEY—GAZIN 509 
less procumbent in 2. megalotis than in B, taylori, and with the depth 
of the jaw below the first tooth somewhat greater to accommodate an 
incisor of greater curvature. Moreover, the coronoid process is 
smaller. 
In the Curtis ranch jaw the last tooth is reduced much as in R&, 
megalotis but the anterior portion of the first lower cheek tooth more 
nearly resembles that in Baiomys, and although the teeth show con- 
siderable wear, the anterior portion of the first seems to be somewhat 
shorter than in either form. The length of the anterior portion of 
the lower jaw, a character stressed by Gidley, cannot be certainly 
determined because the bone is partially broken away from around 
the incisor. The beveled portion of the incisor is much closer to 
the cheek teeth than in other forms but this may have been brought 
about by a backward sliding of the incisor, since the posterior end of 
the incisor is not now covered, although the tooth seems quite secure 
at the present time. In spite of this, the incisor is definitely more 
procumbent than in Rezthrodontomys and slightly more so than in 
Baiomys taylori. The lower margin of the fossil jaw is not entire 
across the incisor below the first cheek tooth and the coronoid process 
is not preserved. 
ONYCHOMYS PEDROENSIS Gidley 
The Curtis ranch Onychomys is now represented by eight jaw por- 
tions, including two that were obtained in 1986; however, none have 
more than two cheek teeth, and in only one is the incisor complete. 
The type is a left lower jaw, No. 10506, with an incomplete incisor 
and the first and third cheek teeth. 
The jaws are noticeably deeper and more robust than in Onychomys 
leucogaster ruidosae and the teeth a little larger, much larger than in 
O. bensoni. Structurally, the teeth are very close to those in 0. J. 
ruidosae, and relatively higher crowned than in O. bensoni. The 
masseteric ridge, as indicated by Gidley, extends farther forward than 
in the Recent form and the last cheek tooth is less reduced; the talonid 
portion, however, is more restricted than in O. bensoni. 
SIGMODON CURTISI Gidley 
In addition to the type, No. 10510, which includes both rami of 
the mandible with all the cheek teeth and a nearly complete incisor 
on the right, the large Curtis ranch Sigmodon is represented by three 
lower portions with two to three cheek teeth each and a left maxillary 
fragment with all the cheek teeth. The upper dentition and two 
of the lower jaws were added to the collection in 1936. 
Sigmodon curtisi is a heavy jawed form distinctly larger than S. 
medius of the Benson fauna, comparing favorably in size of teeth 
with the modern cotton rat, Sigmodon hispidus. The anterior lobe of 
