﻿July, 1896.] 



GENUS LEMMUS. 



37 



Fig. 11. — Enamel pattern of molar 

 teeth, Leinmus lemrmis. (x5.) 



edge of interpterygoid fossa carried forward over (dorsad to) overhang- 

 ing edge of palate (PL II, fig. 14). The anterior edge of the squamosal 

 forms a narrow but distinct shelf-like postorbital process, much as in 

 Synaptomys, but more strongly developed. 



Teeth. — The dentition of Lemmus is essentially the same as that of 

 Synaptomys. The upper incisors are, however, much more slender in 

 proportion to the size of the skull, and are without the peculiar grooves 

 always present in Synaptomys. In the pattern of enamel folding, the 

 only difference between the two genera is that the third transverse loop 

 in the hindermost maxillary tooth is iso- 

 lated by a single reentrant angle in Synap- 

 tomys, and by the contact of two reentrant 

 angles in Lemmus (fig. 11). 



External form. — In external form the 

 species of Lemmus differ very widely from 

 all other microtines except Dicrostonyx. 

 The head is disproportionately large for 



the short thick body,^ while the tail is reduced to a mere rudiment only 

 about two-thirds as long as the hind foot. The feet are highly modified 

 to tit the animals to their fossorial habits. While the hind feet are 

 unusually large and strong, the front feet are even more specialized. 

 The thumb is provided with a large ligulate nail and the fingers are 

 armed with long, sharp claws (fig. 12). The claws are, however, sim- 

 ple in form and are not subject to the periodic changes that occur in 

 those of Dicrostonyx. 



In the alcoholic specimens that I have examined the palms show no 

 trace of tubercles, but the soles bear indications of sev- 

 eral verj^ small and exceedingly rudimentary pads close 

 to the base of the toes. The fur is remarkably long and 

 dense, the palms and soles densely furred, and the tail 

 IDrovided with a pencil of stiff bristle-like hairs longer 

 than the tail vertebr?e. 



General remarlcs. — The species of Lemmus are true 

 lemmings with highly modified skull and external form. 

 With these characters they combine the dentition of 

 Synaptomys without, however, the peculiar incisors of 

 the latter. Lemmus differs from Synaptomys in its highly 

 modified skull and external form as well as in the dental character just 

 mentioned. From Dicrostonyx it is distinguished by cranial and dental 

 characters and by the well-developed external ears (fig. 15), as well as 

 by the simjDle claws and large thumb nail. 



The species of Lemmus at present recognized are L. lemmus (Lin- 

 naeus), Lj. ohensis (Brants), L. scMsticoJor (Lilljeborg), and L. nigripes 

 (True). 



Fig. 12.— Left front 

 foot, Lemmus 

 lemmus (hair re- 

 moved). 



^ This pecuUarity is carried even fvirther in Lemmus tliau in Synaptomys. 



