274 UNIONID2E. 



deposit eggs, which are developed in the interior of 

 their exterior pair of gills. They have been divided 

 into numerous species, every river and lake or pond 

 having its peculiar kind ; but in ponds where there 

 is plenty of food (and a dead dog or cat or fish 

 affords abundance of such material), and where the 

 water is nearly stagnant and seldom disturbed, they 

 become of a large size, with ventricose thin shells, 

 while in more rapid rivers with pure clear water, 

 with very little decomposed animal or vegetable 

 matter held in suspension, they are small, with com- 

 pressed thick shells ; and all intermediate forms and 

 sizes are to be observed. After collecting many 

 hundred specimens from various localities, I am con- 

 vinced that there is only a single species found in 

 this country. 



A most variable species, which appears to assume 

 different appearances under every circumstance, as, 

 for example, the depth, the stillness or motion, and 

 the purity or impurity, or peculiar impregnation of 

 the water in which it happens to be located. 



Mr. Alder considers A. cygnea, A. cellensis Pfeif- 

 fer, A. intermedia and A. anatina Lam., and A. 

 ventricosa Pfeiffer as distinct British species. (Mag. 

 Zool and Bot. ii. 118.) 



Mr. Sheppard, after describing the four species, as 

 he considers the varieties of this species to be, sums 

 up as follows : To bring the specific differences 

 above enumerated into one point of view, M. anatinus 

 is distinguished from M. cygneus by its anterior (pos- 

 terior) area running parallel with its base ; again, 

 from M. macula by the interior area of the latter 





