100 MONOGRAPHS OF NORTH AMERICAN BODENTIA. 



When this exterior triangle is most perfect — most like the antecedenl one — 

 then also it bears the most periled supplementary internal spur; but oftener 

 the two together have an indeterminate contour and a common dentine islet* 

 The last upper molar is the diagnostic tooth of this section of the genus. 

 Certain European species show it exactly as in our forms; hid in North 

 America, as far as is known, no Arvicolce but zanthognathus and the varieties 

 of riparius show the peculiarity. This tooth consists essentially of an ante- 

 rior transverse elliptical loop, one interior lateral closed triangle, two exterior 

 lateral closed triangles, and a long oblique posterior crescent. The ellipse 

 is succeeded first by the first exterior triangle, then by the single interior 

 triangle, then by the other exterior triangle; the long anterior horn of the 

 crescent bends inward to form a second interior saliency ; the long outward 

 convexity of the crescent bears the second exterior triangle upon its back, as 

 it were ; the posterior horn of the crescent curls - inwardly to form a loop that 

 finishes the tooth behind. With endless minor modifications, as matters of 

 individual variability, this crescent is always recognizable and rarely obscure. 

 Generally, it is seen at first glance, as something different from the U-, V-, or 

 Y-shaped trefoils that end this tooth in our other subgenera. Really, of 

 course, it is not a continuous enamel-wall thus stretching crescentic across 

 the tooth; simply, the second (counting from backward) internal reentrance 

 is so deep that it pushes before it a fold of enamel till this touches and gen- 

 erally fuses with the external wall of enamel just behind the second external 

 triangle. It is, in fact, this fusion that produces the last-named triangle itself. 

 (In the other subgenera, the corresponding prism of the tooth is simply the 

 exterior leaflet of the posterior trefoil, opening directly into the midleaf, 

 through lack of the fusion that takes place in riparius.') Now let this second 

 internal reentrance be not quite deep enough to effect this closure, and we 

 have the first modification of the crescent to be remembered, a slighi break 

 in its convexity, just at the posterior angle of the second external triangle. 

 When, as occasionally happens, this break is considerable, the integrity of 

 the crescent is destroyed, and we have a trefoil-like loop simulating that of 

 the other subgenera. But, even in these most obscure cases, we have always 

 found something in the configuration, perhaps not susceptible of definition, 



* This little subsidiary triangle is never, to our knowledge, developed lit all in our other sections 

 of the genus, and therefore, when evident, is a good character ; hut it is very liable to he overlooked — 

 ill fact, it was only after repeated examinations that we verified the nice distinction Baird drew (p. 514) 

 in the matter of this tooth. 



