No. 434-] 



NOTES ON THE IJNIONIDsE. 



difference of another kind : a deep, oblique sulcus on the inside 

 of each valve, in the female, the space occupied by the marsu- 

 pium, so that the sexes can readily be distinguished on the empty 

 shells at least in older specimens. 



It may be repeated here that in younger examples of Lamp- 

 silis, etc., the female shells cannot be distinguished from the 

 males during the first two or three years of lite, and the disten- 



the marsupia. 



The female shell of Tritogoma tube rat lata Barnes, is less 

 inflated posteriorly than the male, and has a flat, thin extension 

 at the posterior end. Here the difference is of another kind 

 than in Lampsilis, a small portion of the branchia, if any, find- 

 ing room in the extended part of the shell, the genus evidently 

 ranging under another group : with Unio (s. str.), Ouadrula, etc., 

 without differentiated marsupia. 1 



In some species of Unio, Quadrula, etc. there are slight, 



more or less marked differences between the male and female 



Species of another group, however, show well-marked differ- 

 ences, e.g., Alasmidtmta tnmcata WAV Wright - and ./. calccola 



Lea. The fact was well known to the older conchologists, and 

 even the late Hy. Moores, three fourths blind, readily discerned 

 the sexes by a touch. The females are more inflated and more 

 voluminous posteriorly, curved down, and having generally a 

 stronger ridge. It is much the same with Strophitus edentulus 



