SYNAPTOMYS 165 



Genus : 2. SYNAPTOMYS Baird. 

 1857. Syiiaplomys Baird, Mamm. N. America, p. 558, 



Genotype. — Synaptomys cooperi Baird. 



Range. — ^North America, from the northern edge of the 

 Lower Austral zone in Virginia (Dismal Swamp) and Kansas 

 (Woodson County) northwards to Alaska, Mackenzie, and 

 Labrador. 



Characters. — General external form nearly as in normal 

 voles. Fur soft and moderately long. Colour dull ; bi'ownish 

 above, paler below. External ears well developed, slightly 

 evident above the fur; in form like those of Evotomys. Hands 

 and feet of normal size and form ; thumb small, jjrovided with 

 a large flattened nail; five palmar and six plantar tubercles; 

 palms clothed between wrist and jaads with long white hairs 

 which conceal the jjollical tubercle; soles clad with shorter 

 and thinner haii-s between the heel and pads. Tail terete, 

 slightly longer than the hind-foot; rather thinly clothed with 

 long stiff hairs which do not com2:)letely conceal the scaly 

 annulations. Mammae, 2 — 2 = 8 or 1 — 2 = 6. 



Skull relatively narrower, more lightly built, and in general 

 less modified than in Leiiimus. Rostrum noticeably deflected, 

 short and broad. Nasals ending anteriorly slightly but dis- 

 tinctly in advance of the strongly curved " opisthodont " upper 

 incisors ; terminating posteriorly in line with the premaxillaries 

 and slightly in front of the orbits. Zygomatic arches lighter 

 and less widely S2)reading than in Leminus, though more expanded 

 than is usual in voles ; the greatest zygomatic breadth falls on 

 the maxillary or fore-parts of the arches and is equal to 61-65% 

 of the condylo-basal length, instead of to about 68% as in 

 Lemmus or to 55-60% as in most voles. Jugals slender, with 

 slightly convex upper borders, their outer surfaces forming 

 nearly vertical planes instead of being abruptly convergent 

 dorsally as in Lemmus. Braincase longer and narrower than 

 in Lemmus, the post-glenoid region less noticeably shortened. 

 Temjioral ridges fusing ^ in old age to form a median inter- 

 orbital crest; their posterior course over the braincase essentially 

 as in Le>nmus, though they are perhaps slightly less salient than 

 in equal-aged skulls of L. leminus. Squamosals very large, 

 essentially as in Lemmus, though relatively a little more widely 

 separated in front than in the latter genus, with long post- 

 orbital crests, which bound the square shoulders of the braincase, 

 and forming the entire floors of the posterior or supratym2)anic 



* In two only (.S'. wrangeli, Metlakatla, British Columbia, B.M., Nos. 

 11.10.2.1 and 13.10.1.1) of the small series of skulls of Synaptomys before 

 me, are the ridges actually fused into an interorbital crest; but in several 

 of the other specimens the ridges are closely appro.ximated. Jlost probably 

 the ridr;es fuse regularly in the older adults of all species of Synnptoinyn; 

 but fully mature skulls of Microtinae are naturally comparatively rare. 



