81 Mr. J. Miers on the Meiiispermacese. 



has only found in it a single plant, a small Alga. This circum- 

 stance seems sufficient to account for the disappearance of 

 Dreissena. Previous to 1780, Cavlsruhe was, no doubt, much 

 smaller than it is at present, and the water was therefore more 

 pure. 



It is not impossible that the small rivulet mentioned by Sander 

 may have become, since 1780, unsuitable for the Dreissena. 

 Perhaps the quality of the water may have changed, or an in- 

 crease in the number of water-fowl or fishes may have destroyed 

 the Dreissena. Mr. James Bryant, the discoverer of Dreissena 

 in England, used it as bait in fishing for perch in the Thames. 

 The circumstance that Dreissena has not been observed in the 

 loess of the Rhine is not a proof of its absence from that wide- 

 spread deposit : I believe neither Unio nor Anodonta has hitherto 

 been detected in it. Nor, in my opinion, can the silence of 

 writers be regarded as a proof of the non-existence of Dreissena 

 in the west of Europe previously to 1 824. The freshwater shells 

 were so little investigated before the time of C. Pfeiffer and 

 Rossmassler, that even Unio tumidus was not known out of 

 Denmark until 1825. U. pictorum and U. hatavus were chiefly 

 known to Schroter and preceding conchologists from Nuremberg 

 colour-boxes. 



The alleged migration of Dreissena seems to be rather too 

 sudden, if not too swift. In 1824 it was noticed in the Thames, 

 in 1825 in the Niemen, and in 1826 in the mouth of the Rhine. 

 Its appearance is more like that of Miastor, which was first ob- 

 served at Astrakhan : as soon as the attention of naturalists was 

 called to it, Miastor was found in every place where it might be 

 expected to occur. Dreissena may owe a great deal of its rapid 

 distribution to commerce, or perhaps to the pontoon-trains of 

 the armies of Napoleon ; but I still deny that there is any proof 

 that it was introduced everywhere in this century, principally 

 between the years 1824 and 1828. 



XIX. — On the Menispermacese. 

 By John Miers, F.R.S., F.L.S. &c. 



[Continued from p. 29.] 

 37. DiPLOCLISIA, 



This genus was proposed by me, in 1851, for a small set of 

 plants of which the Cocculus macrocarpus, W. & A., is the type. 

 It difi^ers from Cocculus, Nephroica, and Holopeira in habit and 

 the structure of its putamen and seed. These differences are so 



