422 EXTERNAL FORM OF SHELLS. 



polygonus of Lamarck, are all varieties of one species.* 

 Murex magellanicus, when found in smooth water, is covered 

 with large acute foliaceous expansions ; hut the same shell 

 living in rough seas is without any such expansions, and 

 only cancellately ribbed. In such situations it seldom 

 grows to a large size ; but when it does so, it becomes very 

 solid, and loses almost all appearance of cancellation. Tri- 

 ton maculosus is very widely spread over the ocean in 

 different temperatures and different kinds of seas ; it con- 

 sequently offers a multitude of varieties both in size and 

 surface, all gradually passing into each other, and most pro- 

 bably produced by the operation of the foregoing causes. 

 Indeed, a vast number of merely nominal species have been 

 formed from the habit, too prevalent among conchologists, 

 of describing from single specimens, or even from several 

 individuals brought from the same locality, which would 

 never have been considered as distinct had collectors kept 

 in their cabinets a series of specimens found under different 

 circumstances, and studied, on the coasts where they are 

 found, the variations which shells undergo. 



Those shells which are attached to rocks, either imme- 

 diately by their outer surface, or by the intervention of a 

 beard, are most acted on by these causes : thus the Anomiae 

 found in protected places are thin and transparent, while those 

 which inhabit exposed situations are thick and nearly as 

 opaque as the shell of an oyster ; and the under valves of the 

 Crania? which are affixed to the branches of coral are very 

 thick and solid, while those that adhere to the Pinna? and 

 other flat shells are so thin as to have been overlooked by 

 conchologists, who have repeatedly described their upper 

 valve as a species of Patella. 



Boring shells are greatly influenced in regard to their 

 size, thickness, and form by the hardness or softness of 

 the rock in which they are found : thus the specimens of 

 Pholas dactylus found in the soft rock of Salcombe, are 

 large and thin, and are covered with beautiful, regular, 

 arched scales ; while those found in the hard rock are 

 small, irregular, thick, with a very wide anterior gape and 

 large dorsal valves, and closely wrinkled externally, but 

 almost or entirely destitute of scales : and the Saxicava?, 

 found in hard limestone, are often curved and otherwise 

 distorted, in order to avoid the harder parts of the rock 

 during the process of boring. 



* " Varietates conchyliorum exclusi numerosissimas, Murices tamen 

 frondosos admisi, quamvis inter se nimis affines." — Linnaeus, Si/st. Nat. 

 1216. 



