262 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



being found on the shore and the other dredged. Helclon pellucidum, 

 when found on the leaves of Laminaria is thin, with bluish rays, 

 but when it crawls down into the roots to live it becomes very thick, 

 and usually loses the colour rays. Dwarf shells, again, thick and 

 strongly formed, are often found on exposed coasts where the surf 

 beats strongly ; and some Littorina, which are confined to more or less 

 brackish water, have thicker shells where the water becomes less salt. 

 In South Australia two races of Littorina unifasciata are said Ho occur: 

 the first, living on rocks, moist during the ebb-tide, is long, slender, 

 and bluish-white ; the second, living on dried and higher places, is 

 short, with a light and dark grey chequered pattern. Area tetragona, 

 as found dwelling in old shells in the Channel Islands, is much 

 swollen, but where it inhabits crevices of rocks, as in the south-west 

 of England, it is compressed and the surface is worn. 



In rivers Unionidse have usually heavier shells than in lakes, and 

 where the stream is turbulent the teeth are stronger and the muscular 

 impressions deeper. Some species seem better able to form shell than 

 others ; for example, in the granitic parts of South-East Bavaria, TJnio, 

 Neritina, and Ltthoglyphus have ^ thick shells, while Anci/lus, which 

 lives with them, has a very thin shell. 



Land-shells living in decayed leaves are usually thin, a result which 

 has been attributed to the difiiculty of obtaining lime. This presence 

 or absence of lime has an effect on the size of the shell, and molluscs 

 have been seen gnawing not only one another's shells but even their 

 own * in the endeavour to procure it. 



Some molluscs have, naturally, flexuous shells, while in others this 

 may occur abnormally, due, no doubt, to some failure in secretion. 



Sexual dimorphism plays an important part in the size and shape 

 of shells, notably in such forms as Unio, etc. 



Temperature, again, must be considered, as, if it becomes too warm 

 or too cold, the animal is much affected ; several of the species found 

 in thermal springs near Buda-Pesth were found* to be dwarfed. 



We may mention, as one of a number of similar cases, the deformities 

 of Zimncea, etc., noticed by Folin in the Lake of Ossegor, where the 

 sea had broken in, abnormal surroundings, with the usual result of 

 abnomial shells, following. 



Gibbosity of the body- whorl is well known in Limn^ea and Planorhis, 

 and has been ascribed to periodicity in growth. 



The variations of LimiKea peregra are said* to fall into natural 

 groups, the long-spired being found in running water, the short-spired 

 in lakes and ponds, while the strongly built forms dwell on the edges 

 of large bodies of water or in turbulent streams, and those from hot or 

 cold springs or great depths have generally a thin shell. 



' Haacke: Zool. Anz., vol. viii, p. 504. 



2 Clessin: Nachrbl. Deutsch. Malak. Ges., 1874, p. 87. 



3 Kobelt: Nachvbl. Deutsch. Malak. Ges., 1872, p. 44. 

 * Hazav : Mai. Blatt., ser. ii, vol. iii, p. 7. 



5 Cf. Taylor: Journ. Conch., vol. vi, p. 284. 



