62 



BULLETIN 171, UNITED STATES NATIONAL MUSEUM 



Hodomys. Of these tlie most readily determinable characters are to 

 be found in the last lower cheek tooth. In old individuals, showing 

 well-worn teeth, tliis tooth, in fossil as well as in hving genera, has but 

 two reentrant angles, one on the inner, the other on the outer face of 

 the crown. In Neotoma these reentrant valleys are oppositely placed 

 and tend to divide the crown surface into two subequal areas. In 

 Hodomys and Parahodomys the reentrants alternate, the inner being 

 well in advance of the outer one, tendmg to form a more or less S- 

 shaped enamel pattern on the triturating surface. However, the 

 latter genera are readily distinguished from each other by the fact 

 that in Parahodomys the reentrant valleys are directed almost at right 

 angles to the longitudinal plane of the tooth row, while in Hodomys 

 they slant well forward. In young mdividuals with slightly worn teeth, 

 additional distinctions are apparent in that there is present a third 

 reentrant fold in tliis posterior molar in all three genera, but this fold 

 is pecuharly situated in the fossil form. In Parahodomys it occurs on 

 the posterior internal lobe opposite the large external fold and extends 

 downward to a point slightly below the middle of the crown. In 

 Neotoma and Hodomys the third reentrant fold is on the anterior lobe 

 of the outer side. It occurs only very near the summit of the tooth in 

 Neotoma, but it extends down to about the middle of the tooth crown in 

 Hodomys. 



Table 19. — Measurements {in millimeters) of mandible and inferior dentition of 

 Parahodomys spelaeus and Hodomys alleni 



Measurement 



Total length of jaw, condyle to incisor border 



Depth of symphysis - 



Depth of ramus at point between Mj and Mj 



Height of condyle above level of cheek tooth row 



Length of cheek tooth row measured at alveolar borders... 



Ml, anteroposterior diameter 



Ml, transverse diameter .-. 



Mj, anteroposterior diameter 



M:, transverse diameter - 



Mi, anteroposterior diameter 



Ms, transverse diameter 



PaTahodomys spelaeus 



U.S.N.M. 



no. 12040 



(type) 



9.5 



3.2 



2 



2.8 



2.1 



2.4 



2 



U.S.N.M. 

 no. 12037 



30 

 6.4 

 7.3 

 7 

 9.5 



3 

 2.4 



2.5 

 2 



Hodomys alleni 



U.S.N.M. 



no. 91525 



(Biol. Surv.) 



28.7 

 4 

 6.6 



10 



10.8 

 3.C 

 2.3 

 3.5 

 2.6 

 2.9 

 2.1 



SYNAPTOMYS cf. COOPERI Baird 



The anterior portion of a single lower jaw, U.S.N.M. no. 7774, in 

 the Cumberland Cave collection, carrying the incisor and first molar, 

 represents a species of Synaptomys that corresponds in size and is not 

 otherwise clearly distinguishable from S. cooperi. In the many speci- 

 mens of the hving species of this genus examined the triangles of the 



