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MONOdUAIMIH OK NOIJTII AMERICAN kODENTIA. 



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When lliis pxterior triangle is most perfcet — most like tlio antecedent one — 

 tiien also it l)ears the most perlect 8iii>|)lementary interniil s|)nr; l)iit oftener 

 tiie two logellier have an iiuletenninate contour and a common dentine islet.* 

 The lust upper inolar is the diagnostic tooth of this section of the genus. 

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

 America, as tiir as is known, no Arvicola but xanthognathus and the varieties 

 of riimrius show tiu; peculiarity. Tliis tootii 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 succeede<l 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 Ijcnds 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 ahcays 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 i« not a continuous enamel-wall thus stretching crescentic across 

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

 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 i.s, 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 ret-ntrunce be not quite deep enough to effect this closure, and we 

 have the first modification of the crescent to be remembered, a slight 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 xubaidiary triangle is never, to our knowledge, developed at all iu our other soutions 

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

 in Tact, it was only after repeated exiit.iinations that we verified the nice distinction Balrd drew (p. 514) 

 in the mailer of this tuoth. 



