LYMN.-EIDyE OF NORTH AMERICA. 



of spiral impressed lines (pi. Ill, fig. F). These costse 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 

 i? repeated among the land shells, where it is notably developed in the 

 Polygyras. 



EPIDERMIS OR PERIOSTRACUM : The epidermis of the Lymnaeids 

 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 (caper at a} 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. 



COLOR : 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'ha.s 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 

 Lymnseas are marked spirally instead of longitudinally, producing a 

 peculiarly marked surface. 



2. EXTERNAL APPEARANCE OF THE ANIMAL. 



Plate III, figures H-K. 



Lymnsea 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- 



