LYMN^.ID^E OF NORTH AMERICA. 3 



The Sculpture (9) in the Lymnaeas 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 Lymnaeas, while the 

 latter includes nearly all of the larger species. 



1. Longitudinal growth lines, (pi. Ill, 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 costae, 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 Lymnaeas as to cause the erection of two genera 

 for their reception (Pleurolimmza 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 1 writes as follows concerning this feature 

 of the sculpture of Lymnaeas : 



"The partially or wholly malleated surface so often met with in 

 the Limnaeids 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- 



^roc. U. S. Nat. Mils., XXIV. p. 291. 



