PLEISTOCENE VERTEBRATES FROM CUMBERLAND CAVE 63 



molars are more tightly closed than in the fossil specimen. This is 

 the only difference observed, but if it could be verified in a large nimi- 

 ber of specimens it might constitute a species distinction. 



SYNAPTOMYS (MICTOMYS) species 



The subgenus Midomys is also represented in the Cumberland Cave 

 collection by a single specimen, a broken lower jaw, U.S.N.M. no. 

 7773, with complete dentition. No characters were observed by 

 which the fossil form could be distinguished from the hving species, 

 S. (M.) borealis. This species is currently recognized to contain all 

 the races within the subgenus Midomys. However, all the lemming 

 mice of this group are at present Canadian and Alaskan in distribu- 

 tion (Howell, 1927, p. 19), extending down into northern Washington 

 and northern New Hampsliire in the United States. Although the 

 fossil appears to demonstrate the presence of this modern boreal spe- 

 cies in the Pleistocene of Maryland, its occurrence so far outside of the 

 present distribution suggests that a distinct species may be represented. 

 Additional miportance is to be attached to this occurrence in furnishing 

 the fu'st Pleistocene record of Midomys (Gidley, 1913b, p. 96). 



MICROTUS (or PITYMYS?) cf. INVOLUTUS (Cope) i« 



Figure 35 



The voles are very sparsely represented in the Cumberland Cave 

 collection. Seven fragmentary specunens, one a part of a palate with 

 the two anterior cheek teeth on the right side, the others broken 

 mandibles, comprise the lot. 



The material is clearly microtine in character but is apparently 

 distinct from North American hving species. The root portion of the 

 lower incisor in the Cimiberland Cave form passes between the lower 

 portions of the penultimate and ultimate molars as in voles, and the 

 cheek teeth are without roots in those specimens where this character 

 can be observed. However, the pattern of the first lower cheek tooth is 

 somewhat simpler than in the living pine-mice of the region. This 

 tooth, presumably Mi, is characterized by a posterior loop followed 

 forward by three closed triangles, a pair of confluent triangles opening 

 into an anterior loop. In these characters the fossil jaw resembles 

 Pitymys but unlike the form living in the same region at the present 

 time, P. pineiorum, the anterior loop of Mi in three of the six jaws is 

 without reentrant angles. Two specimens have shallow reentrant 

 angles on both inner and outer surfaces, nearly as in P. pineiorum, 

 and one has a shallow groove on the antero-internal surface of the 

 anterior loop (fig. 35). Furthermore, the posterior surfaces of the 

 triangles in the lower cheek teeth are shghtly more convex, and isola- 

 tion of the triangles is not so marked as in specimens of P. pineiorum. 



i« Cope, 1871, pp. 89-90, fig. 16. 



