194 MONSTROSITIES OF LITTORINA RODIS. 



and from, them I take the following : The shells were found in 

 muddy or shady ditches in which the soil was of a black or peaty 

 nature. The shells were not so solid as usual and more transparent. 

 There were a few weeds present in the water, but no vegetation 

 growing on the shells except " des mousses microscopiques." Some 

 of the specimens, both regular and distorted, were albinos, and he 

 remarks that the " mousses " were not invariably present on such 

 specimens. None of the other species Planorbis Complanatus, 

 Limnaea Palustris, &c. found with them were distorted or albino. 

 Thus we must notice at once that certain species may be affected, 

 while other species living in the same spot and under the same 

 conditions are not. 



The causes which have produced these forms are either internal 

 or external. They are either due to some defect in the animal 

 itself or are attributable to the influence of its surroundings. I 

 will now deal with the internal causes suggested, and first I will 

 take hybridism. These shells are certainly not hybrids since they 

 exhibit only the characters of Littorina Rudis. Jeffreys and 

 Thomson certainly did notice the coupling of specimens of L. Rudis 

 with others of L. Obtusata at Weymouth, and the same has been 

 noticed in Ireland. A dwarfed and stunted race of L. OUusata is 

 to be found in the Fleet, but I have not seen it so far up as Langton 

 Herring; its termination is, I think, a little above Chickerell. 

 There are some notes by Mr. Ellsworth Call,* the result of the 

 dissection of many specimens of the American freshwater species 

 Melantlio Rufa and M. Dedsa. and of the examination of the 

 embryos they contained, in which he puts forward the theory that 

 the cause of shouldered and sinistral shells is the crowding together 

 of many specimens while in the embryonic stage. He found that 

 one and a-half per cent, of 1,000 of the embryos in Melantlw Rufa 

 were sinistral, and two to two and a-half of 1,000 in Melantha 

 Decisa ; while from collecting the adult shells he found that only 

 one-tenth of one per cent, survived. He attributes sinistral shells 

 to crowding very early, while shouldered ones are accounted for 

 * American Naturalist, vol. xiv. (1880), p. 207. 



