300 TAYLOR : ON THE VARIATION OF MOLLUSCA. 



peregra from the icy-cold waters of the Pyrenees are thin, 

 delicate and of a reduced size, and we have an instance nearer 

 home in the L. stagnalis from Malham Tarn, an elevated body 

 of water in Yorkshire, which produces a very thin and dwarfed 

 variety of that species. 



The most favourable temperature or that most specially 

 advantageous, and in which Z. stagnalis receives its greatest 

 development, is about 77°. Under favourable conditions, the 

 periods of growth are longer in duration, or the growth itself is 

 quicker than in ordinary cases. This will account for the 

 gigantic size of specimens from some localities. Under less 

 favorable conditions, the checks in growth are more numerous, the 

 growth period not so prolonged or the growth not so rapid and 

 there results therefore, a smaller form. Semper has shown in 

 the case of Limncea stagnalis that if the young shells are too 

 crowded, or have a too limited supply of water in which to live, 

 even though food be supplied in abundance, their growth is 

 dwarfed, and such dwarfing cannot be compensated for after- 

 wards by any especially favourable conditions or treatment, as 

 the shell is constructed on a more diminutive scale. 



These facts enable us to account in some cases for the 

 production of dwarfed varieties and it is probable that amongst 

 others the small var. distinguished by M. Jules Colbeau as var. 

 aquarii, had its origin in some of the causes I have mentioned. 



Another effect of cold is blackness or melanism of the 

 naked species. Herr Simroth when treating of A r ion ater and 

 Limax maximus has shown that in these species this character 

 is developed by cold, and redness by warmth, especially when 

 these conditions are applied for a few weeks in spring, which 

 is the period of most rapid growth and development. 

 Confirmatory evidence of the conclusions of Simroth that 

 the Arion ater is a northern or cold form and Arion rufus a 

 southern or warmth variety is afforded by Limax maximus, which 

 in its northern form oi cinereo-niger is almost wholly black, but 

 in the genial climate of Italy it develops a series of brilliantly 



J.C, v., April, i888. 



