PLEISTOCENE VERTEBRATES FROM CUMBERLAND CAVE 57 



that opens obliquely outward and backward through a notch that 

 distinctly separates the hypoconid from the outward extension of the 

 posterior rim of the basin. Paraconid, hypostylid, and entostylid of 

 P4 well developed. Basins of the molars broad and evenly concave. 

 Space between protoconid and hypoconid relatively wide with very 

 distinct valleys on each side of the well developed hypostyle. Inner 

 rim of the cheek tooth crowns interrupted by a median, sharply 

 defined notch. 



Family GEOMYIDAE 



PLESIOTHOMOMYS POTOMACENSIS Gidley and Gazin » 



Figure 31 



Type. — Portion of left ramus of mandible (fig. 31), U.S.N.M. no. 

 8166, with all cheek teeth. Incisor not preserved beyond margin of 

 alveolus. 



Generic characters. — Anterior margin of symphysis of mandible 

 very abrupt. Pit on lingual side and at base of ascending ramus 

 shallow, as in Thomomys. Masseteric crest nearly straight and ex- 

 tending high on mandible anteriorly. Lower molars oval in cross 

 section and with enamel plates on anterior and posterior surfaces. 



Specific characters. — Size near that in average adult specimens of 

 Thomomys julvus. Mandible robust with deep symphysis. Incisor 

 relatively wide with anterior surface smooth and rounded. Lower 

 cheek teeth relatively narrow transversely. 



The pocket-gopher material in the Cumberland Cave collection 

 referred to this species includes four other lower jaws, one of which, 

 U.S.N.M. no. 12049, possesses a complete lower dentition. 



Comparison. — These specimens present several interesting charac- 

 ters that seem to separate them clearly from both Thomomys and 

 Geomys. The mandibles are remarkably deep and the symphysial 

 portion is turned upward rather abruptly, causing the nearly straight 

 anterior margin of the symphysis to form a sharp angle with the 

 inferior margin of the ramus. Also, the noticeably straight masseteric 

 crest rises anteriorly at a higher angle from the lower border of the 

 jaw than in living forms, making the distance between the anterior 

 extremity of the masseteric crest and the ventral Hmit of the symphysis 

 relatively great. 



The dentition of these specimens is unique in combining characters 

 of both Thomomys and Geomys. The incisor has its anterior surface 

 rounded as in Thomomys but is relatively wide. P4 has its enamel 

 plates as in both Thomomys and Geomys, but the columns of this 

 tooth are well rounded and the internal reentrant fold is compressed, 

 suggesting Geomys. The external fold is more open. The molars 

 are widely oval in shape, perhaps even less compressed antero- 



" Gidley and Qazln, 1933, pp. 354-366, flg. 7. 



