﻿July, 1896.] 



GENUS SYNAPTOMYS. 



Skull. — The skull of Synaptomys (fig. 9 and PI. I, figs. 12, 13) is moder- 

 ately broad, flat, aud massive, much less so than in the other Leomni. 

 Eostrum short (nasal bones about one-fourth occipito-nasal length) and 

 strongly deflexed; zygomatic arches not broadly flaring as in Lemnus 

 and Dicrostonyxj^ though more so than in the voles; middle portion of 

 zygoma very slightly expanded, the outer surface nearly vertical ; brain 

 case not greatly broadened or flattened, and seldom if ever consi)icu- 

 ously ridged or furrowed; interparietal with rounded corners, the 

 antero-posterior diameter more than half the transverse diameter; 

 pterygoids short; interpterygoid fossa about one-sixth basilar length 

 of skull; posterior border of bony palate ending nearly as in t3'i)ical 

 Microtus. (See p. 26, PI. II, fig. 5, and fig. 7, p. 27.) Front edge of 

 squamosal forming a narrow, shelf-like i:)ostorbital process. 



Teeth. — Anterior faces of upper incisors with distinct longitudinal 

 grooves. Lower incisor terminating posteriorly a little in front of the 

 hinder edge of the back molar. Throughout its length each mandibu- 

 lar incisor lies wholly on the inner (lingual) side of the molar series. 

 (PI. Ill, fig. 1.) 



The molars are all rootless. The upper molar series is about one-third 

 the basilar length of skull, the lower series slightly less. The enamel 

 pattern (figs. 8 and 10) is characterized by the great depth of the outer 

 reentrant angles in the maxillary teeth and of the inner reentrant 

 angles in the mandibular teeth. Of the maxillary teeth m 1 and m 2 

 show no important peculiarities of form except that the outer reentrant 

 angles cut across to the enamel of the extreme inner side, a feature 

 shared by Lemmiis alone. The posterior upi^er molar, however, like 

 that of Lemmus^ differs widely from the corresponding tooth in all other 

 Microtinw. It is formed of four transverse looi^s. The first and second 

 of these loops are isolated by two deep reentrant angles on the outer 

 side of the tooth, while the third is formed by an equally deei) depres- 

 sion on the inner side. The reentrant angles and closed triangles on 

 the inner side of the mandibular molars are greatly developed at the 

 expense of those on the outer side. In the subgenus Mictomys the lat- 

 ter wholly disappear except in the last tooth. This has a reentrant 

 angle near the middle, but no closed triangle. 



External form. — In general appearance Synaptomys resembles the 

 Ilicroti much more closely than it does the Lemmi^ a fact which has 

 given rise to the rather inappropriate names ' lemming- vole' and 'false 

 lemming.' The species of Synaptomys are thick-set microtines with 

 large heads, ears that just appear above the moderately long fur, short 

 tails, and small feet. In color they are all dull brownish, darker on the 

 back, paler on the belly. The palms and soles are tuberculate, as in 

 the voles. 



General remarks.— Synaptomys differs from all the other genera of 



1 Tlie ratio of zygomatic breadth to basilar length is approximately 70 iu Synaptomys, 

 75 in Lemmus and Dicrostonyx, and 65 in Microtus. 

 16933— No. 12 3 



