MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 34 1 



procured by Mr. Jeffreys in Billingsgate Market, and one by 

 Mr. Rich, which you will see in my drawer to-night; and in 

 Purpura lapillus, which is as common or even more universal 

 in its distribution and numbers, only two specimens — one 

 picked up by Mr. Bean's little grand-daughter at Scarborough, 

 and the other by Mrs. Stebbing in North Wales, which latter we 

 had an opportunity of seeing last year in Dr. Norman's collection. 



As might be expected, the examples of normally sinistral 

 shells known to show dextral varieties are rarer than the opposite 

 condition, as they themselves are so much fewer in number. 



Moquin Tandon, in his work on the European Land and 

 Freshwater Shells, gives five instances, and Jeffreys one, viz., 

 Clausilia rugosa var. dexirorsa, which is also one of the species 

 mentioned by the French author. To this list I can add one, 

 which you will see this evening, viz., a dextrorsal specimen of 

 Balea perversa, from the collection of the Rev. Revett Sheppard, 

 to which he had given the MS. name of Turbo hlandi. 



As to the causes of variation among shells themselves they 

 are almost infinite in number, and I can only glance at a very few. 



Among these are abundance or scarcity of food, influencing 

 the size and thus producing geographical races of giants and 

 dwarfs \ abundance or scarcity of lime, influencing the thickness 

 or thinness of the shell. Warmth or cold may affect the size in 

 either direction, some shells being much larger in warm and 

 others in colder areas. A remarkable instance of this is given 

 by Johnston, who states that the specimens of Littorina neri- 

 toides found on the southern slope of Plymouth breakwater, and 

 therefore fully exposed to the sun, are twice the size of any 

 found on the northern face. Exposure to the fury of the ele- 

 ments ; the marine univalves which have to resist the effects of 

 the surf are generally squat in form, while those that live in 

 deeper and quieter regions are more elongated and elegant in 

 in shape ; this especially applies to different specimens of the 

 same species, 



A similar condition may be seen in land shells, as in the 

 large race of Helix tiemoralis found on the Irish Aran. The 



