LYMNAIDZ OF NORTH AMERICA. 0 
of spiral impressed lines (pl. III, fig. F). These costae may be either 
convex, in which case the spiral lines are impressed, or concave, causing 
the spiral lines to form pseudo-threads. The distance between these 
spiral lines varies considerably in the different species as well as in 
the same species and on the same individual. This beautiful sculpture 
is repeated among the land shells, where it is notably developed in the 
Polygyras. 
EpIDERMIS OR PERIOSTRACUM: ‘The epidermis of the Lymneeids 
is usually very thick and in many cases may be peeled off when the 
shell becomes dry. It is deposited by the margin or collar of the mantle. 
Breaks made in the shell are repaired by the visceral mantle, but they 
lack the epidermal covering, which is secreted only by the edge of the 
mantle. In one species (caperata) the epidermis is hispid and stands 
erect in the spiral lines, causing the surface to assume the aspect of 
having rows of hairs. This may be observed only in fresh specimens. 
The color of the shell is in the epidermis and becomes chalky and life- 
less when this is removed. 
Cotor: The normal color of the shell is light yellowish horn. 
The color is affected very largely by the environment. Those species 
which live among aquatic vegetation generally have clear, translucent, 
yellowish shells, while those which inhabit muddy bottoms where there 
is an excess of decaying animal or vegetal matter have dark-colored 
or even black shells. Bulimnea megasoma has a brownish shell which 
is streaked with yellow, green and mauve. Some species from upper 
Michigan have a shell of a white, waxy color which is beautifully trans- 
lucent. Dark brown is characteristic of the species of certain localities, 
while some individuals have a more or less pearly luster. When the 
animals live in a shallow pond or other body of water in which the 
bottom is covered with decaying vegetation, a shell is produced which 
is longitudinally striped in zebra fashion. the light zones being narrow 
and the darker ones wide. In Rock Run, near Joliet, some of the 
{LLymnzas are marked spirally instead of longitudinally, producing a 
peculiarly marked surface. 
2. EXTERNAL APPEARANCE OF THE ANIMAL. 
Plate III, figures H-K. 
Lymnea is provided with a broad, oval or elongated foot which 
is squarely truncated or broadly rounded before, rounded behind, and 
much flattened at the margins. The head is separated from the foot 
and occupies the vela area (which is retained from the larval stage), 
which is short and wide, ranging in form from semi-circular to quad- 
