MURID^— ARVICOLIN^E— SYNAPTOMYS. 231 



In Synaptomys, the front upper molar is composed of five prisms, and its 

 crown of as many closed dentine islands surrounded by enamel walls: — an 

 anterior loop across the tooth, then a small internal triangle close to the first, 

 then a larger external triangle widely separated from the first by a reentrance 

 reaching the inner side, then a small internal triangle close to the last, then 

 a postero-external loop, the latter separated from the last by a very deep 

 reentrance similar to the other reentrance. The middle upper molar has four 

 dentine islands: — an anterior loop reaching across the tooth, then a large 

 external triangle that also extends across the tooth, then a small strictly 

 internal triangle, and then a directly posterior loop. The last upper molar 

 has likewise four dentine islands, all of which are rather transverse loops 

 than triangles, and really reach from side to side of the tooth; the first of 

 them is separated from the next, and this second from the third, by a deep 

 external reentrance, while the fourth is separated from the third by a similar 

 internal reentrance. All this of the upper molars is exactly as in Myodes. 



The front under molar, like the front upper one, has five dentine islands: — 

 an anterior loop somewhat trefoil-shape or triangular reaching across the 

 tooth, then a large internal lateral triangle, then a very small external lateral 

 triangle, then a large triangle reaching nearly across the tooth, and finally 

 a posterior loop reaching quite across. But the external lateral is so minute 

 and inconsiderable, and the two internal saliencies between the anterior 

 and posterior ends of the tooth make such large triangles, that the tooth 

 might be thus described: an irregular anterior trefoil, a regular posterior 

 crescent, with two large triangles between, one or both of which may bear 

 a small spur, isolated or not, upon its exterior angle. The middle under 

 molar has four triangles, of which the first one and last two are large and go 

 quite across the tooth; the other one is a minute external appendage to the 

 first one, hardly distinguishable. In this'tooth only is there a slight difference 

 from Myodes, which has an additional triangle anteriorly, making five in all. 

 The back lower molar is almost exactly like the middle one, having the same 

 four triangles; it is merely a little smaller, but the minute external triangle 

 may not be obvious, leaving apparently three. Both the middle and lower 

 one may be described as being in general shape like the letter M, with the 

 two legs and middle saliency of the M internal and acute, and the two horns 

 of the M obtuse and external. 



The molar series, as wholes, are of about ordinary length, measuring 

 0.26 to 0.30 in length; their length, relative to the skull, is as 0.28 to 1.12, or 



