274 
UNIONIDiE. 
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 j 
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 j 
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 | 
