294 ' CONCHIFERA. 



Unio margaritifera. Drap. 132. t. 10. f. 1719.; 



Pfeiffer, i. 116. t. 5. f. 11. 

 Unio sinuata. Lam. H. vi. 70. ; Pfeiffer, ii. 33. ; 



t. 7. f. 4. 

 Unio margaritifer. Rossm. Icon. i. 122. f. 72. 74., 



ii. 21. f. 129. 



Unio elongatus. Nilson, 106. n. 2. 

 Unio elongata. Mich. Compl. 113. t. 16. f. 29. 

 Roissyi. Mich. Compl. 112. t. 16. f. 28. ; 



Proceed. Brit. Assoc.for 1838. 

 Unio margaritifera, forma Roissyi. Forbes ; Malac. 



Mon. 44. 



Alasmodon margaritifer. Fleming, B. A. 417. 

 Young, with small posterior lateral teeth, like* 



Unio. 



Unio riparia. Pfeiffer, t. 5. f. 13. ?? 

 margaritifera. Pfeiffer, ii. t. 7. f. 1. 

 Var. kidney-shaped, lower edge elongate, bent in 



making a concave line. 



Inhab. mountain rivers with a rocky bed. 



The young specimens are oblong, without any con- 

 traction in the lower edge ; and in some rivers, the 

 shell appears to retain this appearance in its adult age ; 

 and it has hence been regarded as a species by Mi- 

 chaud and others, under the name of U. Roissyi. But 

 in general, as they enlarge, they become much eroded 

 about the umbones, and the lower margin is more or 

 less drawn in. In this state, it is the U. elongatus of La- 

 marck. I do not know whether the erosion of the beak 

 has any thing to do with the contraction of the lower 

 edge, but all the oblong regular specimens I have seen 

 have the periostraca of the beaks only slightly worn ; 

 while, on the contrary, all those that have the beaks 



