168 MICROTINiE 



loops are separated by the very deep second outer fold, the 

 first inner infold being but slightly develo])ed. In the lower 

 molars the outer infolds may be deep enough to close triangles 

 (subgenus Synaptomys, Fig. 73, 16) ; or so weakly developed 

 that the teeth consist of transverse loops and have crenulate 

 instead of serrate outer borders (subgenus Mictomys, Fig. 73, 

 2&, 3&). 



Subgenera and species. — Eleven species, comprising thirteen 

 forms, are at present included in the genus ; but future work 

 will probably reduce the number of species and increase the 

 number of geographical races. The known forms may be arranged 

 in two subgenera, viz., Synaptomys Baird and Mictomys True, 

 distinguished as follows : — • 



Mammae, 1 — 2 = 6. 



Lower cheek-teeth with closed triangles. 

 Rostral part of skull very stout; palate without long 

 posterior sj3ine. 



Subgenus tSynaptomys Baird. 

 Mammae, 2—2 = 8. 



Lower cheek-teeth without closed triangles. 

 Rostral part of skull slender ; palate with long spinous 

 process behind. 



Subgenus Mictomys True. 



Subgenus : SYNAPTOMYS Baird 



1857. Synaptomys Baird, Mamm. N. America, p. 558 (genus). 



1896. Synaptomys Miller, N. Amer. Fauna, No. 12, p. 34 (subgenus 



of Synaptomys); Merriam, Proc. Biol. See. Washington, 10, 



1906, p. 57. 



Genotype. — Synapto)nys cooperi Baird. 



Range. — -Eastern North America, from Virginia and Kansas 

 northwards through the eastern United States into eastern 

 Canada and westwards to Minnesota. 



Characters. — Externally as described under the genus. 

 Mammae, 1 — 2 = 6. 



Skull with remarkably heavy rostrum ; palate nearly as in 

 Microius, without long posterior spine. Incisors very large and 

 broad ; grooves of upjjer incisors usually well defined and 

 externally placed. Mandibular cheek-teeth with deep outer 

 infolds and closed triangles substantially as in Lemmus. 



Geographical differentiation. — ^Four forms referred to three 

 sjDccies are at present recognized ; but according to Rhoads 

 (Proc. Acad. Nat. Sci. Philadelphia, 1897, pp. 305-307) all 

 should be regarded as subspecies of S. cooperi. Only one of 

 these forms is well represented in the material before me, and I 

 can therefore come to no decision ; but after carefully studying 

 the literature and my material I am inclined to think that Rhoads 

 is right. 



