508 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
The skull portion of Nerterogeomys persimilis is near that of 
Geomys texensis in size but with a shorter and somewhat shallower 
rostrum and the ventral surface anterior to the cheek teeth not ex- 
tending so far above the plane of the cheek teeth. The upper incisors 
are slightly smaller but bisulcate as in Geomys texensis. ‘The cheek 
teeth are also a little smaller and relatively narrower than in Recent 
Geomys. The anterior column of P* is much narrower than in 
Geomys material, suggesting Zhomomys, and the reentrants between 
the columns of this tooth are more open than in Geomys, though not 
so wide open as in Zhomomys. Of particular interest is the presence 
of enamel across the posterior wall of P* as in Thomomys, but not 
Geomys. According to Merriam ** enamel occurs on a portion of the 
posterior wall of this tooth in the Mexican and Central American 
forms Heterogeomys, Macrogeomys, Zygogeomys, and some Ortho- 
geomys. Enamel was also noted on the posterior wall of P* in 
Plesiothomomys ? orientalis (Simpson)** from the Pleistocene of 
Florida, and as in J. persimilis the rostral portion of P. ? orientalis 
was observed to be somewhat depressed. 
The lower jaw of XV. persimilis appears to be more robust than that 
of NV. ? minor, and the masseteric ridge is much better defined, but 
as in that species the mental foramen is situated below the anterior 
extremity of the masseteric crest rather than in front of it as in 
modern Geomys and Thomomys. The lower cheek teeth in XY. per- 
similis are small and relatively narrow as in J. ? minor but the anterior 
column of P, appears smaller and more nearly circular in outline. 
The anterior surface of the lower molars is without enamel, com- 
parable in this respect with Geomys and differing from Thomomys 
and Plesiothomomys. 
BAIOMYS BRACHYGNATHUS (Gidley) 
The type and only specimen of this species is a right lower jaw, 
No. 10501, with all the teeth present and well worn. The form seems 
most nearly comparable to Baiomys taylori as indicated by Gidley, 
but with a more reduced last lower molar. The last molar is reduced 
in material of Retthrodontomys as well as Onychomys, and although 
the form seems clearly distinct from Onychomys the differences from 
Reithrodontomys are not so obvious. 
Jaws of Reithrodontomys megalotis are comparable in size to those 
of Baiomys taylori but with a somewhat more reduced last lower molar 
and slightly better developed anterior portion of the first lower cheek 
tooth, with the anterior pair of cusps more sharply separated from 
the second pair lingually. Also, the lower incisor is more curved or 
33 C, H. Merriam, North Amer. Fauna 8, p. 79, 1895. 
4G. G. Simpson, Amer. Mus. Nov., No. 328, pp. 6-7, 1928. See also J. W. Gidley and 
C. L. Gazin, U. S. Nat. Mus. Bull. 171, p. 59, 1938. 
