﻿July, 1896.] 



GENUS PHENACOMYS. 



41 



The brain case is, liow- 



FiG. 16.— Side 

 Phenacomys. 

 adult. (x3.) 



view of molars, 

 (a) young, (b) 



Bony palate not terminating in- a thin-edged shelf continuons between alveoli 

 of posterior incisors. 

 Feet not specially modified. 

 Thumb with a small pointed nail. 

 Tail longer than hind foot, terete. 

 Fnr not specially modified. 



SltuJl. — Tbe skull of Phenacomys (PI. I, fig. 5) differs very slightly in 

 general form from that of typical Microtus. 

 ever, flatter and more quadrate (but no more 

 so than in the subgenera irtr/iir?^5 ^in&Fitymys), 

 and the zygomata bend down somewhat more 

 abruptly in front. Th e expan sion of the zygo- 

 ma at the region of contact between the malar 

 and the zygomatic pro(iess of the maxillary is 

 rather more abrupt than is usual in Microtus^ 

 but the difference is very trifliii g. The postor- 

 bital processes of the squamosals are slightly 

 more prominent and angular than in Microtus 

 arvalis or M. pennsylvanicns, but scarcely more 

 developed than in M. agrestis, and consider- 

 ably less so than in M. alleni. The audital 

 bullae are proportionally about the same size 

 as or slightly smaller than in Microtus arvalis. 

 They are more globular and less ' subfusiform ' 

 than in the typical species of true Microtus^ but closely resemble those 

 of M. agrestis. The palate (PI. II, fig. 1) is formed essentially as in the 

 members of the subgenus Lagurus (PI. II, figs. 3 and 4). 



Teetli. — The .teeth of Fhenacomys differ in many ways from those of 

 the other voles. In young individuals the molars (tig. 16) are rootless, 

 but by the time the animals are full grown each molar has developed 

 two distinct roots, which, however, remain open until an advanced age, 

 though not so long as in the genus Evotomys. 

 The pattern of enamel folding (fig. 17) is essen- 

 tially the same as that of the voles of the sub- 

 genera Fedomys and Fhaiomys. (See pp. 56 and 

 57.) The differences are to be found in the lower 

 molars where the reentrant angles on the inner 

 side are proportionally deeper and those on the 

 outer side proportionally shallower than in Fed- 

 omys. There is a corresponding difference in the size of the closed 

 triangles on the opposite sides of the teeth. The anterior outer loop 

 in the second lower molar is especially reduced. 



In Fhenacomys the root of the lower incisor runs back between the 

 roots of the second and third molars, and terminates on the outer side 

 of the tooth row in the ascending ramus of the jaw, at about the level of 

 the middle of the posterior molar, and distinctly below the dental fora- 

 men. (PI. Ill, fig. 2.) While exactly this condition is not found else- 

 where except in Evotomys^ it is somewhat closely approached in Fiber. 



Fig. 17.— Enamel pattern of 

 molar teeth, Phenacomys 

 celatus. (x5.) 



