STANDEN : THE ZEBRA-MUSSEL. 85 



reports it from the Shropshire Union Canal near Beeston Castle; 

 from the Grand Trunk Canal at Lostock Gralam ; and the Peak 

 Forest Canal at Marple. In 1863 Mr. Leo H. Grindon found it 

 swarming in the lodges of some print works at Handforth, where it 

 caused great inconvenience by lining the inside of the water-pipes, but 

 Mr. Oldham could only find a few dead shells there when he visited 

 the place in 1887. Mr. J. W. Jackson found quantities of dead shells 

 in the canal near Poynton, last year, but failed to discover any living 

 ones on such portions of the walls as were accessible. Byerley ('53) 

 and Bellars ('58) both, in their Cheshire lists, record Dreissensia from 

 the Ellesmere Canal. 



At the Annual Meeting of the Conchological Society held at 

 Stafford in 1899, Mr. W. Wells Bladen exhibited a shell of Z'mi'j'^^i-m 

 containing a pearl, taken from the canal near Rugely. The pearl is 

 attached, almost spherical in shape, and has a small protuberance 

 at one side; it is 2 mm. in diameter, and is very fine in colour. This 

 is the first recorded instance of a pearl being found in this mollusc. 



Dreissensia is apparently so nearly allied in form to Mytiltis that it 

 is not surprising the question should have been raised as to whether 

 it is not a marine form which has gradually accustomed itself, as other 

 estuarine species have been known to do, to an admixture of fresh water, 

 and ascended the streams until it became completely habituated to it. 

 This explanation is probably true, for we find that it is reported as 

 st'l' living amongst marine mussels in the Aral and Caspian Seas, and 

 the estuary of the Volga; in both these situations it is, however, only 

 found in such parts as are much impregnated with fresh water. Not- 

 withstanding this instance of its retention of a semi-marine habit we 

 may safely assume that it is now a true freshwater mussel, and conse- 

 quently its development, so widely differing — as I shall presently shew 

 — from all other freshwater Pelecypoda, is deeply interesting. 



The mystery attending the long unknown reproduction and early 

 life history of Dreissensia has been satisfactorily elucidated by Dr. 

 Korschelt ('92), and when his paper on the subject came into the hands 

 of the late Professor A. Milnes Marshall, he was much interested in 

 it, and under his directions I commenced a series of observations 

 with a view to following out those made by Dr. Korschelt, and, if 

 possible, adding a little thereto. I procured a large quantity of 

 healthy Dreissensia from the clean waters of the canal at Newsham, 

 near Preston, biinging them away in situ on stones, etc., and placed 

 them in a number of tanks brought as near as might be to the con- 

 ditions under which the mussels had lived. They did very well in 

 these aquaria, and were interesting creatures to watch, but I think 

 we began too late in the season. However, our observations, so far 

 as they went, coincided with Dr. Korschelt's, and we proposed to go 



