286 TAYLOR: VARIATION OF LIMN^A PEREGRA. 



one season in a pool near Geneva, had a curious malformation of 

 the base of the columella, and that this peculiarity was coincident 

 with an extraordinary abundance of Hydra vlridis in the same 

 pond, and that the disappearance of the Hydra the following 

 year, was also concurrent with the disappearance of the 

 peculiarity in the shell. 



Generally speaking, it is believed by most conchologists 

 that lakes or pools favour the relative increase in breadth and 

 decrease in length of the shell, owing to the more rapid enlarge- 

 ment of the whorls increasing the comparative size of the body- 

 whorl and diminishing that of the spire, the highest development 

 in this line of variation is Limj^cea burnetii oi hXdex, in which the 

 apex is actually intorted or sunk within the succeeding whorls. 

 Limncea involuta Thompson is a still more striking instance of 

 this peculiarity, but though it is probable that L. involuta is like 

 L. burnetii only an extreme variety of L. -peregra, the fact has 

 not yet been fully and satisfactorily demonstrated. 



The opposite condition, comparative increase in length 

 and decrease in breadth, caused by the elongation of the spire and 

 the more slowly enlarging character of the whorls, thus approach- 

 ing in form Z. palustris and L. iruncatula and exemplified by 

 the varieties microstoma, elongata, &c., is said to be most 

 usually found in flowing waters, and though this is generally the 

 case so many exceptions occur that it is obvious that other less 

 evident and as yet unascertained causes in this as in other 

 circumstances, exercise great influence in modifying the contour 

 of the shells, and what would perhaps be the result of the 

 prominent features of the environment if not counteracted by 

 other less obvious conditions. 



The strong-shelled forms appear from published observations 

 to inhabit the margins of turbulent streams or rivers, and the 

 shores of large bodies of water, where the wave commotion 

 necessitates a robust shell to withstand its force and violence, 

 the varieties lutea, fiuminensis, solida, &c., serving to typify 

 this line of variation, 



J.C, vi., Oct., 1890. 



