THE ZEBEA MUSSEL. 41 



deserves a passing notice from the fact of its being 

 now so generally distributed that it cannot fail to 

 attract the attention of the collector.* It was first 

 noticed in the Commercial Docks at Kotherhithe, 

 and, being able to attach itself by a strong byssus to 

 extraneous substances, there can be little doubt that 

 it was introduced upon Baltic timber. It is now to 

 be met with in canals and rivers in various parts of 

 the country, and, through the New Eiver, has even 

 found its way into the streets of London. Some 

 iron water-pipes which were taken up in Oxford 

 Street were found to be in some places completely 

 lined with these mussels, and the colour of the 

 shells was as bright as if they had been always 

 exposed to the light. In shape, the shell is oblong, 

 with a sharp keel in the middle of each valve, and 

 flattened below, with the end or beak pointed. The 

 colour is a yellowish-brown, transversely barred on 

 the upper part with darker brown, giving it that 



* M. Marcel de Serres is of opinion that the habitat of 

 Dreissena polymorpha was originally marine, from the cir- 

 cumstance of the shells being found in tertiary strata of 

 marine formation. Pallas, by whom the species was first 

 made known, described one variety of it as marine, and 

 another as inhabiting fresh water. 



