SNAILS AND THE SOIL — VAN CLEAVE 277 



LIME SUPPLY 



Mollusks are so intimately dependent upon a lime supply for the 

 fabrication of their limy shells that among fresh-water forms some of 

 the poorest molluscan faunas are encountered in the areas poor in 

 limestone but rich in granite. This correlation is so close that col- 

 lectors of land shells have long recognized limestone cliffs and out- 

 crops as particularly favorable collecting sites for living land snails 

 and streams flowing through limestone or through soils rich in lime 

 as particularly productive of snails and mussels. 



Snails, living and dying in the same place, little influence the lime 

 supply, for the limy material which they use is sooner or later returned 

 to the soil again, and the lime cycle is repeated over and over within 

 and without the shells of the living snails. When the shells disinte- 

 grate, the lime which had been temporarily entrapped is again re- 

 turned to the general store. No quantitative studies have been made 

 to determine if there is any dissipation of the available lime supply 

 in this cyclic utilization of lime by the living organisms. 



The late T. D. Foster (1937) recorded for a flood-plain forest along 

 the Sangamon River in Illinois the presence of 2,630 individuals of one 

 species of land snail (Mesodon thyroidus) per acre. Though there 

 are about a score of species of snails present in this same area, the one 

 species is the most abundant and relatively the most important so 

 far as production and utilization of lime is concerned. On the basis 

 of weights which Foster calculated, about 169 pounds of Mesodon 

 thyroidus are present on each acre of the woodland studied. Of this 

 weight, about 140 pounds are of snail flesh and about 29 pounds are 

 of dry shell. This does not represent the entire annual accumulation 

 but rather the amount present at one time in the shells of living snails 

 as determined by sampling methods. Because of the cyclic fluctua- 

 tions in abundance of all types of organisms, figures of this sort have 

 but relative value, for at another time either greater or less abundance 

 might be demonstrated (Foster and Van Cleave, 1937). However, 

 the presence of the snails is an indicator of available lime supply and 

 in turn their shells after death return lime to the soil as potential 

 material for use by other organisms. 



To a much smaller degree, the shells of zonitoids, pupids, and other 

 minute snails living in grasslands utilize the available lime and return 

 it to the soil, although quantitative studies on the losses in the transfer 

 and by action of ground water have never been determined. 



SHELLS NOT STATIONARY 



In referring to the intimate relationship between the molluscan 

 fauna and their physical and biotic environment, attention should be 



