THE NAUTILUS. 



Voi,. XXV. MAY, 1911. No. 1 



THE CARDINAL TEETH OF PISIDIUM. 



BY V. STERKI. 



It has been asserted by earlier writers that Pisidium amnicum 

 Miill. and virginicum Grnel. have two cardinal teeth in the right 

 valve, and mainly on the strength of that character, Clessin has es- 

 tablished the group Fluminina. Some authors have ascribed that 

 feature to other species of Pisidium, or even to all, indiscriminately and 

 without examination. But the whole thing is based on a misappre- 

 hension ; the fact is that the two Pisidia, named have only one cardinal 

 tooth in the right valve (and two in the left, like all others), and con- 

 sequently the feature holds good not only for Pisidium, but for the 

 whole family, Spheeriida. It might be added that if those species 

 had really two teeth, Fluminina would mean a distinct genus, at 

 least. 



In young, and even many half-grown specimens, there is plainly a 

 single cardinal tooth, strongly curved. With advancing age there 

 appears an indentation in the middle (above), as the anterior and 

 posterior parts are growing more. They grow also in thickness, even 

 the thinner anterior often becomes grooved, and at the same time the 

 whole tooth becomes more curved, resp. angular, resembling the two 

 in the left valve (inversely). But a careful examination will show 

 that the two shanks are connected. 



On the other hand, in mfiny specimens of P. compressum Pr., vari- 

 abile Pr., etc., there is a more or less marked indentation in the 

 middle of the right cardinal. 



