INTBODUCTIOtf. XVli 



a thousand miles from the coast ; Corbicula and Sphcerium are 

 generally sand or mud dwellers ; while Pisidium largely affects 

 running streams where there is still considerable vegetation in 

 which to lurk. 



As would be naturally supposed the waters of the plains and 

 lower mountain-slopes are the most productive of molluscau life, 

 though the writer was able to record some years ago the presence 

 of Limncea and Pisidium in Thibet at an altitude of 14,500 feet,* 

 this probably establishing a record for the elevation at which 

 mollusca are known to exist, though it is only fair to state that 

 the stream in which they were found was fed from a warm spring 

 and was only completely frozen over in the coldest months 

 (February to March). 



The extraordinary adaptability, in adverse circumstances, of 

 certain of the delta genera is remarkable, being able, as they are, 

 to thrive equally well in either brackish or almost salt and per- 

 fectly fresh water as occasion arises, as is also the power of some 

 species of both Gastropods and Pelecypods to sestivate buried 

 deep down in almost dry mud during times of drought. 



To illustrate the powers of endurance of certain species it may 

 here be stated that Limncea pereger has been known to occur in a 

 spring in Iceland having a normal temperature of 40 Centigrade, 

 while a species of Paludestrina has been taken also in a warm 

 spring with a temperature as high as 50 Centigrade. 



As far as is known, the freshwater Gastropoda are usually 

 vegetable feeders, though some specimens of an African proso- 

 branch (Cleopatra), kept alive recently by the author, eschewed 

 oatmeal and all other vegetable food provided for them, prefering 

 instead to devour the periostracum of one another, this canibalistic 

 action, however, was probably produced by the sudden change of 

 climate and environment to which they were subjected. 



The Pelecypoda, having once passed the parasitic embryonic 

 stage, are equally vegetarian in their habits. 



5. Movements, Locomotion, and Dispersal. The progress of the 

 fluviatile Grastropods is necessarily slow as they glide along on 

 the "foot" across the mud, rocks, or up the aquatic plants on 

 which they live, this gliding motion is caused by the alternate 

 contracting and expanding of certain portions of the lower surface 

 of the foot, several contractions and expansions being frequently 

 in operation in different parts of its " sole " at the same time ; in 

 the Pelecypods, however, the causes of the mode of progression 

 are rather different, the foot, it is true, is also used to propel the 

 creature on its way, but the gliding motion of the Gastropods 

 gives place to a jerky movement caused by the foot being ex- 

 tended, then swollen by the blood being, as ifc were, pumped into 

 it, thus giving it power in its swelled condition to obtain a grasp 

 of the object or exact spot which is to form the end of the step, 



* Eec. Ind. Mus. Calcutta, iii, 1909, pp. 115-116. 



