64 Our British Snails 



sub-order Heteromya {i.e. with adductor or 

 closing muscles not equal) ; to the family of 

 MytilidcB (or mussels) and the genus Dreissensia 

 (named after a Dutch conchologist). 



Dreissensia polymorpha is a triangular, boat- 

 shaped, bivalve, supposed to have been intro- 

 duced with Russian timber (as was also probably 

 Hydrobia Jenkinsi). It is gregarious, and at- 

 taches itself to objects by a byssus like our marine 

 mussels. Shell yellowish-brown with wavy pur- 

 plish lines, wrinkled in the line of growth. Com- 

 mon in the New River, and has been found in 

 iron water-pipes in Oxford Street. 



All our shells have varieties (many an 

 albino or white form), and the collection and 

 distinguishing of these varieties, which in some 

 species are numerous, adds much to the interest 

 of the collector. In addition there are also the 

 variations in size or markings w^hich can hardly 

 rank as varieties. Inasmuch as none of our 

 shells are peculiar to our country (which 

 is from the natural history and the geological 

 point of view only a detached portion of the Conti- 

 nent), it may be well to warn young collectors 

 that if they receive shells from the Continent, 

 mere varieties are there often named as separate 

 species and variations considered as definite 

 varieties. This is especially the case with Heli- 

 cella virgata. 



