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1 58 



M()X()(ll!APn8 OP NORTIT AMERICAN IIODENTIA. 



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Icclli. 'PIk- iwculiarify of (lie l)iU'U iiitpcr molar i.s not sliarcd by any other 

 Nortli Aincrican section ol" Arvicola, tliotigli re-occurring in Evofomi/x. 



'I'Ik! buck lo\v(;r nioiar consists of three prisms, the crowns of vvliich 

 form tiirec nearly transverse spherical triangles, with their greatest convexity 

 posterior; the inclosed dentine islets reach across the tooth (/. c, there is no 

 median lino of enamel dividing off lateral triangles on cither side, as is the 

 case with all the other teeth; though sometimes, when the middle islet is 

 very sliarjj-aiigled and a little oblique, it gives rise to a partial enamel-line, 

 al)utting against the wall of the back islet). As elsewhere throughout the 

 genus Arvicola, the tooth is abruptly narrower than the antecedent one. 



The middle lower molar has n posterior transverse crescentic ellipse, or 

 spherical triangle, with its greatest convexity backward, its dentine islet 

 reaching across the tooth. The rest of the tooth is of four alternating lateral 

 closed triangles: the first internal, the second external, the third internal, the 

 fourth or front one antero-cxternal. Thus there arc in all three saliencies on 

 each side and two reiintrances on each side {three internal reentrances, if 

 a nick between the antero-external and next internal triangle be counted). 

 'J'he median zigzag of enamel effects perfect closure of all these triangles, 

 starting from the front of the back loop forward and inward to form the base 

 of the next lateral triangle, then shooting forward and outward, then forward 

 and inward, then forward and outward. This tooth is much shorter and a 

 little narrower than the next one. 



The front lower molar is the longest of all the teeth; its crown is 

 exactly that of the middle lower mo\f\v plus one more internal lateral triangle 

 and the large anterior trefoil. Thus there are in all three external saliencies 

 (two of them being closed lateral triangles) and four internal saliencies (three 

 of them being closed triangles), without counting the lobes of the anterior 

 trefoil, which give another saliency on each side; and likewise there are three 

 external reentrances and four internal reentrances, without counting the nicks 

 of the anterior trefoil, which give another reentrance on each side. The 

 posterior loop and the several lateral triangles of this tooth are pretty con- 

 stant, but the anterior trefoil varies interminably in precise size and shape. 

 It would be useless to attempt to give its endless minor modifications. But 

 we should note that cither lube of this trefoil may closely simulate an addi- 

 tional lateral trFangle; that this is particularly the case with the outer lobes, 

 and, when it is acconipanicd l)y a forward extension of the median line of 



