Dr. 0. A. L. Morch on the History of Dreissena polymorpha. 83 



which contains ten figures of species of the genus Unio, inclu- 

 ding two or three of U. batavus. It therefore seems to me im- 

 possible that the Pinna fluviatilis could be U. batavus, as Dr. 

 Gysser supposes. 



H. Sander, Professor at the Gymnasium lUustre in Carlsruhe, 

 who died at the early age of twenty-eight, was certainly not a 

 professed conchologist ; but several volumes of his sufficiently 

 show that he was a person of unusual intelligence, and had a 

 considerable knowledge of natural history. As he chiefly occu- 

 pied himself with the fauna of his native country (entirely an 

 inland one), it is not surprising that he regarded Anodonta as 

 the type or analogue of Mytilus, and that he did not place his 

 new shell in the same genus. After much study, investigation, 

 and comparison, he came to the conclusion that the shell in 

 question must belong to the genus Pinna of Linne, to which 

 genus he likewise refers M. edulis, having seen the latter during 

 his journey through Belgium. Sander gave his shell the name 

 of fluviatilis on account of its having a different habitat from 

 M. edulis. Dreissena is the only freshwater shell that has any 

 resemblance to Pinna. Sander^s description relates to the co- 

 lour only, viz. : outside dark green ; inside blue, with yellow 

 stripes ("Streifen"), which, when held towards the light, are 

 iridescent. The last expression is translated by Dr. Gysser as 

 "perlmuttern^^ (nacreous), although that word is used by Sander in 

 the same page for the inside of Anodonta, and not for his shell. 

 The rays in Unio are visible mostly on the outside. The fulgu- 

 rate markings of Dreissena are not always present, nor are 

 they easily perceptible in full-grown specimens ; but may they 

 not have been meant by '* Streifen^^ ? Some specimens are 

 stated by Sander to attain the size of 2 inches (Rhenish?). I 

 have not oberved any larger than 45 millims., or nearly 1 J inch. 

 Perhaps Sander^s measurement was taken from memory or judged 

 by the eye. Two inches would not be an extraordinary size for 

 any Unio except U. batavus ; and that size is mentioned by Sander 

 as unusual and only attained in certain specimens. 



The disappearance of marine .Mollusca from places which they 

 used to inhabit is by no means infrequent : it has been explained 

 by Mr. Jeffreys in his work on British Conchology. Similar 

 instances among the freshwater Mollusca may fairly be pre- 

 sumed. Some years ago, Limnaa peregra was common in a small 

 pond in the park of Frederiksberg ; it is now no longer to be 

 found there, although other species remain. Apus productus is 

 said to have disappeared from places where it was formerly 

 abundant. Dr. von Martens informs me that the water of the 

 little river Alp is very impure, in consequence of its receiving all 

 the drainage of the town of Carlsruhe ; Professor Alex. Braun 



