﻿36 



NORTH AMERICAN FAUNA. 



[No. 12. 



this difference is constant or otherwise, it is, liowever, impossible to 

 say. 



General remarks. — Mictomys was first described as a full genus, but 

 the characters on which it rests are of no more 

 than subgeneric importance. The group is 

 distinguished from true SynajHomys by the 

 slender rostrum and incisors, slightly different 

 form of bony palate, crenulate outer border of 

 Tig. 10.— Enamel pattern of lower molars, and probably by the number of 



molar teeth Synapto.nys in- ^^^^^^.^ ^^^^^ 

 nuitus. (x5.) 



Four siDecies of Mictomys have thus far been 

 described, Synaptomys inmiitns (True), >S'. wrangeli Merriam, 8. dalli 

 Merriam, and ^S'. truei Merriam.^ 



Genus LEMMUS Liuk. 



1795. Lemmas Link, Zool. Beytriige,, I, Pt. II, p. 75, 1795. Type by elimination Mas 

 lemmas Linn. 



1811. Myodes Pallas, Zoogr. Rosso-Asiat., I, p. 172, 1811 (part). 



1877. Myodes Cones, Monogr. N. Am. Rodentia, p. 237, 1877, and most subsequent 

 authors. 



Geographic distribution of type species. — Arctic region in Asia and 

 eastern Europe. 



Geographic distribution of genus. — Arctic region in both hemis- 

 pheres. 



Essential characters : 



Upper incisors without grooves. 



Lower incisors Avith roots on inner (lingual) aide of molars. 

 Molars rootless. 



Enamel pattern as in Synaptomys. 

 Feet highly modified. 



Palms and soles without well-developed tubercles. 

 Thumb with large flattened ' strap-shaped ' nail. 

 Tail shorter than hind foot, terete. 

 External ear small but well developed. 



Shull. — The skull of Lemmus (PI. I, fig. 6) is perhaps the most highly 

 modified in the family Microtiim. The rostrum, like that of Synapto- 

 mys, is short in proportion to the length of the skull (nasal bones con- 

 tained about three and one-half times in occipito-nasal length), the 

 dorsal profile bent abruptly downw^ard. Zygomatic arches very ab- 

 ruptly and broadly flaring, each expanded near the middle into a wide, 

 strongly oblique plate.^ Brain case broad, flat, and subquadrate in 

 outline, but dwarfed in appearance by contrast with the large zygomata. 

 Pterygoids short (about as in Synaptomys). Bony palate terminating 

 essentially as in Synaptomys, but lateral pits very deep and anterior 



1 See Merriam, Proc. Eiol. Soc. Washington, X, p. 61, 1896. 



2 These plates may be nearly 5 mm. across in the widest part. 



