17 



Dreissena polimorpha (The Zebra Dreissenai). 

 Pallas. 



Figures 7 and 8. 



The form of this shell bears a striking resemblance 

 io ihe common sea-mussel ; its shape, however, varies 

 considerably in the length and breadth, some specimens 

 being much shorter and more ventricose than others. 

 It also never attains the size of our marine species 

 (Mytilus edulis). The colour of the shell is a greenish- 

 brown, with chocolate zigzag markings (somewhat like 

 the striping of the Zebra). These stripes are very ap- 

 parent in certain specimens, and scarcely to be recog- 

 nized in others. The internal colouring of the shell is 

 a blueish-white. The epidermis is glossy. Professor 

 Forbes considers a large specimen to be If inches long, 

 and one inch broad. The largest specimen dredged 

 from the lake at Highfield House measures one inch 

 and eleven-sixteenths by one inch and one-sixteenth; 

 another, more ventricose, 1| inches by one inch. This 

 shell grows to a much larger size in this lake than in 

 any other locality in the neighbourhood of Nottingham. 

 All the specimens found in the Lenton Canal are small. 

 The habit of the Dreissena is rather singular, being 

 always found adhering to stones, wood, or to other 

 c2 



