4 THE CHICAGO ACADEMY OF SCIENCES. 



tions. The habits of these mollusks include, if not properly speaking 

 burrowing, wallowing or submersion, and moving as they do with 

 somewhat of a rotating motion, this, combined with the moderate im- 

 pact of the surrounding matter, contributes to produce the malleated 

 or dinted surface, which frequently exhibits a somewhat spiral arrange- 

 ment." 



The writer believes that Dr. Steam's theory is correct, as it is 

 borne out by the ecological study of such species as palustris, rcflexa 

 and bulimoides techella. These species, with some of their varieties, 

 are inhabitants of ponds and streams which dry up during a part of 

 the year, compelling the animal to burrow in cracks, crawfish holes 

 and under debris of various kinds. The newly foimed shell is fre- 

 quently added while the animal is wallowing in a few inches of water 

 and thus takes the form of the surface upon which it is resting, which 

 is usually very uneven. It is a significant fact that a malleated shell 

 taken from the environment of a summer dry pond and placed under 

 uniform conditions in an aquarium where there is an abundance of 

 water, continues its shell growth without a trace of malleation. It is 

 also noticeable that the malleated individual lives in more or less shallow- 

 ponds and pools among all sorts of debris, while those individuals 

 without malleation and with smooth shells live in deeper waters of a 

 less transient nature. Lymnseas from rivers and large lakes are seldom 

 malleated, as may be seen in most individuals of catascopium and stag- 

 nalis. Dr. James Lewis believed that malleation was due to a rapid 

 growth in warm water, which caused the shell to solidify unevenly. 1 

 Dr. Lewis seems to have laid great stress on the temperature of the 

 water, instead of on the mechanical action of the environment, which 

 seems, from present data, to have been the primal cause of this feature. 

 Temperature does influence growth in other directions, however, as 

 may be seen in almost all species which have a wide northern range. 



2. Spiral sculpture. The presence of spiral, more or less wavy, 

 incised grooves has been frequently noted by many observers. This 

 sculpture is characteristic of the majority of the species of Lymnseidae, 

 and, in fact, may be called typical. It is developed to a high degree 

 in catascopium, the shell of which appears as though engraved with 

 a tool. Stagnalis also exhibits the same appearance, although the lines 

 are finer. Under the compound microscope these so-called spiral lines 

 are seen to be caused by the elevation of the growth lines to form an 

 infinite number of small costae which are interrupted more or less regu- 

 larly, these breaks between the ends of the costae giving the appearance 



»Am. Jour. Conch. VI, p. 85. 



