LYMNAZIDA OF NORTH AMERICA. 3 
The ScutprureE (9) in the Lymnezas falls under two rather dis- 
tinct types, which, however, intergrade more or less in some species. 
First, the longitudinal lines of growth and second, the combination 
of these growth lines with more or less strongly marked spiral lines. 
The first type is largely confined to the smaller Lymnzas, while the 
latter includes nearly all of the larger species. 
1. Longitudinal growth lines. (pl. II, fig. E.) These may be 
so fine and inconspicuous as to cause the shell to appear smooth until 
examined with a lens. In many shells, however, these lines are raised 
to form more or less equidistant ribs or coste, as may be seen in some 
specimens of megasoma, palustris, emarginata, etc. A single specimen 
of megasoma in the collection of the Chicago Academy of Sciences has 
this form of sculpture so marked as to appear almost costate (23097) 
on the last whorl. This characteristic is developed to such an ex- 
tent in two groups of Lymneeas as to cause the erection of two genera 
for their reception (Pleurolimnea and Polyrhytis). In not a few cases 
these ribs form extensive bulgings, causing the shell to assume a patho- 
logical aspect. The cause of this costate appearance is said by some 
to be due to a varying food supply, but may be equally as well ascribed 
to peculiarities of environment, such as alkalinity or salinity of the 
water, excessive cold or the presence of some acid in the fluid medium, 
which also causes many shells in certain localities to be eroded at the 
spire. The peculiar sculpture called malleation, which causes the sur- 
face to assume the aspect of hammered brass, is also due to environ- 
mental causes. That this malleation is due to causes which may be 
of a transient character is proven by the existence of numerous speci- 
mens (megasoma, palustris), in which this characteristic is developed 
on only a part of the shell, the upper whorls and the last portion of 
the body whorl being without this feature. 
Dr. R. E. C. Stearns? writes as follows concerning this feature 
of the sculpture of Lymnezas: 
“The partially or wholly malleated surface so often met with in 
the Limnzids regardless of altitude or the salinity of the water, and 
less frequently in the Physas, is explainable by the character of the 
lake or pond bed in which these dinted forms occur. The character 
of the bottom, even in a pond of limited size, often exhibits very con- 
siderable differences in the matter of compactness or density; alluvial 
mud, clayey mud, clay or sand, with fine or coarse gravel intermixed 
with fragments of aquatic plants and plant stems in varying propor- 
PPro). Ss. Nats. MUS,, cx V. p. 291. 
