58 



BULLETIN 171, UNITED STATES NATIONAL MUSEUM 



posteriorly than is common in Geomys. In Thomomys these teeth 

 have their hngual portion tightly compressed and their buccal surface 

 distinctly flattened and usually grooved longitudinally. However, 



Figure 31.— Plesiothomomys potomacensis Gidley and Gazin: Left ramus of mandible, type specimen 

 (U.S.N.M. no. 8166), lateral and occlusal views. X2. Cumberland Cave Pleistocene, Maryland. 



the molars in the fossil specimens have enamel developed on both 

 anterior and posterior surfaces as in Thomomys. In Oeomys only the 

 posterior surface of these teeth has an enamel plate. 



Table 17. — Measurements {in millimeters) of viandihle and dentition of Plesiotho- 

 momys potomacensis and P. orientalis 



Measurement 



PUsiothomomys potomacensis 



U.S.N.M. 

 no. 8166 

 (type) 



U.S.N.M. 

 no. 12049 



Plesiothorno- 

 mys orienlalis 



A.M.N.H. 



no. 23441 



(type) 



Superior dentition 



Length of diastema between alveoli of I and P4- 



Anteroposterior diameter of upper incisor 



Transverse diameter of upper incisor 



P<, anteroposterior diameter at occlusal surface.. 



P*, transverse diameter of anterior lobe — 



P«, transverse diameter of posterior lobe -.. 



M', anteroposterior diameter at occlusal surface. 

 M', transverse diameter - .-- 



12 



U.8 

 1.7 

 2.2 

 1.6 

 1.8 

 1.1 

 1.9 



Mandible and inferior dentition 



Distance between lowest point on dorsal surface of mandi- 

 ble anterior to Pi and ventral extremity of symphysis — 



Length of diastema between I and P« 



Length of lower cheek-tooth series measured at occlusal 

 surface - - - — 



I, anteroposterior diameter 



I, transverse diameter - 



P«, anteroposterior diameter at occlusal surface... 



Ti, transverse diameter of anterior lobe.. 



P*, transverse diameter of posterior lobe 



Ml, anteroposterior diameter at occlusal surface.. 



Mi, transverse diameter 



Ml, anteroposterior diameter at occlusal surface 



Mj, transverse diameter 



Ml, anteroposterior diameter at occlusal surface - 



Mj, transverse diameter - 



2.1 



2.6 



L3 



2 



1.4 



2 



1.5 



1.9 



1.5 



1.7 



8.5 

 8.6 



7.6 

 2.3 

 2.2 

 2.8 

 1.4 

 2.1 

 1.5 

 2.2 

 1.6 

 2.1 

 1.6 

 •1.8 



' Approiimate. 



