SPHARIUM. 155. 
4. S. LACUSTRE (zahabiting lakes) Miiller. P\. IX., f. 4. 
Round; shoulders sharp; more compressed than .S. corneum ; thin, 
glossy, semi-transparent, grey ; regularly striate concentrically ; deaks 
tipped with a small protuberance (ca/icalation); ligament narrow $ 
hinge strong. L. 73mm. B. Io mm. 
Found in similar habitats to S. corneum but is more 
local, though widely distributed. In Ireland it “appears 
to be absent in the north, and in the south and east it 1s 
decidedly rare.” Glasgow and the neighbourhood of 
Edinburgh is the extent of its range in Scotland. With 
regard to the capping of the umbones—the special 
character of this species—-Mr. Taylor (‘Monograph of 
the L. and F. W. Moll. of the B. Isles,’ part I., p. 43). 
says: “Caliculation is apt to take place to a noticeable 
extent, only in those species in which the embryonic 
shell is of comparatively large size and somewhat globular 
shape, and the succeeding shell growth does not continue 
on the same plane.” It is thinner and more compressed 
than S. corneum, and its round outline differentiate it from 
the oblong S. pallidum. The valves close very perfectly 
which enables it to survive a drought. My finest speci- 
mens were taken alive from the surface of the mud of a 
small pond which had been dried up for two months. 
Var. I. brochoniéna (Bourg.). Much larger and flatter. 
Var. II. rotunda (/ef.). Rounder and flatter. 
Var. III. ryckhbltii (Norm.). Small, shoulders rounded 
off, beaks prominent. 
Var. IV. ovdlis (Fér.). Somewhat oval, with in- 
distinct caliculation. 
