342 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



animals are protected by massive shells from the heavy winds 

 and rain coming straight from the Atlantic. Here, again, nature 

 shows that there is more than one means of obtaining the same 

 result, viz., the protection of the individual under the same 

 conditions. The shells of Helix ericetorum, which live in 

 myriads on the same rocks, are much thinner, more delicate and 

 elastic than those found on our southern Downs ; if heavier, 

 they would more probably be broken, as the plants to which 

 they cling are tossed about by the wind, while the massive shell 

 and the powerful foot of H. neinoralis allows it to resist destruc- 

 tion. Again, the massive Trochus and the delicate Homalogyra 

 are found in the same situation. 



Among bivolves Jeffreys draws attention to the parallel 

 case of Mactra solida and its variety elliptka, forms which 

 exhibited so striking a difference that until recently all con- 

 chologists regarded them as distinct species. Again, there can 

 be no better illustration of this than in the common Anomia, 

 which, when living a quiet and protected life, has a thin and 

 semi-transparent shell, while in other circumstances it resembles 

 a small oyster from the thickness of its valves. Similar causes 

 affect not only the size but the sculpture of the shells — those 

 exposed to rough usage show little beauty of surface, while those 

 enjoying quieter conditions may be richly adorned with spines 

 or foliaceous scales, as in Pinna rudis among bivalves, out of 

 which many so-called species have been manufactured, simply 

 from differences in the spines and sculpture of the shell ; and 

 Purpura lapillus var. iinbricata among univalves. 



Does age affect the size of shells ? This is a question 

 which I cannot answer definitely. I am inclined to think not. 

 The Anodons procured by Mr. Heathcote have been thought to 

 owe their immense size to advanced age, but specimens of 

 Unios found in quiet lakes, and bearing marks of extreme age, 

 are of no more than the average size. M. Picard published a 

 paper in the Bulletin of the Linnean Society of Normandy in 

 1840 on the changes induced by age in the Unios. Owing to 



J.C., vii., Jan. 18^4. 



