BIVALVES. 
27 
called Alasmodonta margaritifera, is markedly 
distinct from the other species; in its superior 
size, often five and a half inches long and two 
and a half wide, and one thick,—it is thus pro¬ 
portionately much longer and more compressed; 
in its strong and pitch-black epidermis, and in 
the adult specimens by the posterior tooth being 
obsolete. The umbones are extensively eroded, 
and the valves are narrowed in the middle. It 
loves to lurk among the gravel and small stones 
in the shallows of quick-flowing rivers or moun¬ 
tain streams. It is found in the North of England, 
North and South Wales; near Ross, in the Wye; 
Devonshire and Cornwall; in the rivers flowing 
from the Scotch Highlands ; and in many of the 
North and South Irish rivers. 
It burrows its shell somewhat obliquely, a 
small portion of which is thus only exposed. A 
diligent search is required to find it, as by the 
growth of confervas upon the little exposed por¬ 
tion, it cannot easily be distinguished from 
amongst the surrounding stones. 
It is very susceptible to the action of light, 
opening the valves on a hot sunny day; but if the 
sun be overcast they remain closed. Country 
boys wade for them, or take them by thrusting 
the end of a long slender rod into the partially 
open shell, which closes upon it, and the prize is 
thus dragged to shore. 
