﻿July, 1896.] 



GENUS EVOTOMYS. 



43 



Essential characters: 



Upper incisors without grooves. 



Lower incisors with roots on outer side of molars. 



Molars rooted. 



Enamel pattern characterized by approximate equality of reentrant angles. 



m 1 with five closed or nearly closed triangles. 



m 3 with three closed triangles. 



Feet not specially modified. 



Thumb with a small, pointed claw. 



Fur not specially modified. 



Tail longer than hind foot, terete. 



^hull. — The skull of Evotomys (PI. I, fig. 4), as compared with that 

 of the other voles, is characterized by a general weakness and lack of 

 angularity. All the outlines are full and 

 rounded, and the ridges and furrows are slightly 

 developed, even in extreme old age. The in- 

 terorbital region is broader and the audital 

 bullse are larger and more inflated than usual in 

 Microtns and PJienacomys. On the other hand, 

 the zygomata are very slender and scarcely 

 widened in the region of contact between the 

 jugal and the zygomatic process of the maxillary. 

 The mandible also is slender and weak. The 

 bony palate terminates in a thin-edged shelf, 

 continuous between the alveoli of the i)osterior 

 indsors (fig. 7 and PI. II, fig. 10). The structure 

 is very different from that found in PJienacomys 

 and in typical Microtns 



Teeth. — The incisors are exactly as in Phenacomys. The lower incisor 

 runs back along the lingual side of the first and second molars, but 

 crosses the line of the molar tooth row between the second and third 

 molars, terminating in the ascending ramus of the mandible at about 

 the level of the middle of the posterior molar and distinctly below the 

 dental foramen. The molars are rootless in the young (fig. 18), but in 

 the adult each is provided with two distinct roots 

 which eventually become fully closed.^ In one very 

 ohl individual the crowns of the lower molars are 

 completely worn away, so that each root, with the 

 exception of the anterior root of m 3 (which has 

 been shed) stands alone like a simple, round-topped 

 tooth (PI. Ill, fig. 4). The molars are all very 

 narrow and weak, in this character strongly contrasted with the strong, 

 broad teeth of Microtus and Phenacomys. 



Fig. 18.— Side view of molars, 

 Evotomys. (a) young, (b) 

 adult. (x3.) 



Fig. 19.— Enamel pattern 

 of molar teeth, Evotomys 

 gapperi. (x5.) 



' For detailed comparison of the palates of Evoiomijs and Microtus see pages 26-28. 



2 In the original description of the genus Flienacomijs (North Am. Fauna No. 2, 

 p. 30) it is stated that "Phenacomys has genuine rooted molars, not half-rooted 

 molars like those oi Evotomys, which grow from persistent pulps." Evotomys, how- 

 ever, has as perfectly rooted molars as Phenacomys, though the roots do not close so 

 early in life. 



