MUltlD.E— AUVICOLIN.E— AUVICOLA UIl'AKIUS. 



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enanic'l, the l()l)o is cist olF IVoiii the rest, ot" (ho licloil, and traiisfoniicd iiilo 

 an actual closed cxtnrnal triangle, of wliicli, tlieretore, the tooth may show 

 indiiFerenlly two or three. A step further in nioditication is this: — tla; trefoil, 

 after heing partitioned otF as just explained, may (hnelop an external lobe on 

 its niidleaf, and so remain in efTect a trefoil, giving in uW Jive external salieii- 

 cics, viz: one from the posterior loop of the tooth; two from the two closed 

 triangles proper; one from tiie addilioiiul closed triangle; and one from the 

 supi)lementary lobe of the trefoil.* We have not noticed the same thing 

 with the inner leaHct of the trefoil, but presume it nuiy occur. 



The upper molars reverse the pattern of the lower; their transverse loop 

 is in front instead of behind, and the lateral triangles alternate from before 

 backv/ard instead of from behind forward; and the last upper molar ends 

 l)ehind, just as the iirst lower molar begins in front, with a formation not 

 seen on any other teeth. The first upper molar, like the last under molar, 

 is very constant throughout the whole genus; the last upper and first under 

 are our diagnostic teeth for the several subgenera, tiiough the middle ones, 

 both upper and under, furnish collateral characters. 



The front upper molar has in front a transverse loop, sicceeded by two 

 internal and two external alternating lateral closed triangles; the first of these 

 being internal, the last postero-external. The saliencies and reentranccs on 

 both sides are sharp; the saliencies, both external and intenial, are three in 

 number, counting a corner of the anterior loop in each enumeration. 



The middle upper molar likewise consists of an anterior loop succeeded 

 by alternating lateral closed triangles, but here the first is an exterior instead 

 of an interior one. The anterior ellipse usually sits a little oblique, its con- 

 vexity looking inward as well as forward. Regularly there are only two 

 ])erfectly-closed lateral triangles on this tooth ; first an exterior, then an inte- 

 rior, for the last one is as much posterior as interior, and not often regularly 

 triangular. It may be called an exterior triangle, api)cndaged with a 

 posterior lobe or spur representing an imperfect additional internal triangle. 



* This is our view of tue formation of a variable nnmbcr of lateral closed triangles in tUiH tooth ; 

 it is simply a higher (le{,'ree of complexity in the ibids of tbu uutoiior trefoil. In Peilomya and Pili/myi, 

 the reverse, of (oiii complexity, is h<'oii. Hero the median zigzag of enamel does not run so far forward ; 

 the anterior triangles are not fairly closed np, and their openness throws their dentine islets into one 

 dlaniond-Kbaped urea that is nsiiully also continuous with the interior of tho trefoil itself. This is 

 nearly as goo<l a character for distinguishing Pedomyg or i'i(i/m;/« from ony style of .1. riparius a* that 

 afforded by the back upper mn'ar itself; thougl. it is sometimes obscure, wo have never seen the anterior 

 lateral triangles fairly o|>en in riparius, nor completely closed in the other two subgenera. Chilotue, 

 however, is iike ripariiM in this respect. 



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