213 



THE BRITISH NATURALIST 



[October 



expanded one. Mr. Taylor expresses a similar opinion, instancing 

 L. hnvnetti as an expanded form of L. peregrn inhabiting still water, 

 " while the opposite condition, comparative increase in length and 

 decrease in breadth . . . as exemplified by the varieties 

 microstoma, elongata, &c., is said to be generally found in tiowing 

 waters." It is added, however, that many exceptions to this rule 

 prove that other unascertained causes exercise great influence in 

 modifymg the contour of shells. This rule, as regards substance of 

 shell, does not hold good for Aiiodon, which in larger and almost 

 stagnant sheets of water attains its greatest size with a light shell, the 

 smaller and more heavy varieties being usually found in rivers. This 

 is what might be expected, the greater thickness and strength of the 

 shell being necessary to withstand the force of the current when 

 increased by rainy seasons. 



Strong Currents. — Mr. J. B. Bridgman is of opinion that the 

 peculiar breadth of the var. compressa of U . pictoruui is caused by the 

 wash of strong currents, "the current washing away the softer par- 

 ticles of mud, and the shell having harder work to keep itself partially 

 buried in the bottom causes the unusual expansion." The Rev. W. 

 C. Hey attributes the presence of small and malformed specimens of 

 U . titmidus to the effect of water rushing over a dam, and he adds, 

 "rapidly running water is always deleterious to the development of 

 such shells as generally effect still waters." Mr. Taylor says, writing 

 of L. pevegra, "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, fltminalis, solida, &c., serving to typify this line of 

 variation." 



Depth of Water. — Mr. Taylor writes, "Deep water is said to 

 have a similar effect [to the extremes of heat and cold] judging from 

 Nilsson's description of L.halthica [di Y^ix'iety ol L . pevegra] which is 

 said to live at a depth of 24 to 36 feet in the brackish-water of the 

 Baltic, and Mr. W. Thompson, the able Irish naturalist, ascribed the 

 peculiar delicacy of the specimens of the var. laciistris [of L. per&gra] 

 from Lough Neagh, to the circumstance that they habitually lived in 

 the still depths of the lake." 



Food doubtless has a very large share in determining the size of 

 all species of Molluscs, but to produce a large size other favourable 

 conditions, suited to the species are required. Mr. E. H. Rowe read 

 a paper before the Lambeth Field Club some years ago, in which he 

 mentioned taking some extremely fine examples of Succinea putris, 

 giants among the Succinece, Coclilicopa in the same situation was also 

 of large size and beautiful lustre. Examining the other parts of the 



