218 LAND AND FRESH-WATER MOLLUSKS. 
(Pl. X., fig.124).—This is the most elegant and 
at the same time the smallest of our fresh-water 
mollusks. The shell bears some general resem- 
blance to that of P. albus; but its minute size, 
dull appearance, and, above all, its sculpturations, 
serve to distinguish it. It may be described as 
—flat, rather concave in the middle, lower side 
rather convex, thin, pellucid, dull, ight-brown 
or grey; the whorls three, the last one enlarged, 
strongly marked with transverse ridges, which 
are more marked’in young individuals; the 
diameter is two-twelfths of an inch. 
The species is known by another name, P. 
imbricatus, perhaps more characteristic than the 
one adopted, which, however, has the priority. 
P. nautileus is widely distributed, but is not 
a common species. 
The readiest plan to secure this species and 
other minute fresh-water shells, is to collect the 
conferve and plants on which they live, place 
them in a basin, and pour warm water on them ; 
the animals will then relinquish their hold and 
fall to the bottom. 
PLaNoRBIS NiITIDUS—(the Shining Coil Shell) 
(Pl. XI., fig. 131).—This and the following 
Species are associated together from the glossy 
aspect of their shells, and from the last whorl 
being very large in proportion to the rest, and 
more or less embracing them. The shell of P. 
