EXTERXAL FORM OF SHELLS. 421 



which, having been attached to a plank covered by a number 

 of Eahmi and Serpula% has the upper valve marked with 

 prominences, exactly agreeing in shape with the substances 

 concealed beneath the under one. The edges of the valves 

 of Barnacles being very closely affixed to the surface of the 

 substances to which they are attached, it appears that thej 

 not only assume the form of the larger i^rominences, such as 

 the ribs and spines of a shell, but also the most minute 

 differences of its surface. A Barnacle in my collection, 

 which had been fixed to a Scallop (Pecten suborbicularis), 

 has not only the ribs of the latter marked across its valves, 

 but the whole surface of the prominent part of the valves 

 is covered in addition with miiiute rugosities, produced by 

 the small projecting scales which cover the surface of the 

 ribs of the Pecten, whilst the articulating portion of the 

 valves is smooth, as in the common state of the species. 

 In another similarly ribbed specimen the articulating por- 

 tions are also ribbed like the rest of the valves ; and in a 

 third, which was found on a piece of roughly planed and 

 loosely textured wood, the surface of the valves bears an 

 exact resemblance to the grain of the wood on which the 

 specimen was attached. 



The thickness, the roughness, and the smoothness of the 

 surfaces of shells appear to depend, in a great measure, 

 on the stillness or agitated state of the water which they 

 inhabit. The species of our own coast afford abundant 

 instances of this : the shells of Buccinum undatum and B. 

 striatum of Pennant have no other difference than that the 

 one has been formed in rough water, and is consequently 

 thick, solid and heavy ; and the other in the still water of 

 harbours, where it becomes light, smooth and often coloured. 

 In the same way the specimens of Purpura lapillus, which 

 inhabit sheltered situations, are covered with small arched 

 scales, whilst those found in exposed places are thick and 

 rugose. Lamarck, not being aware of this circumstance, 

 considered the specimens in the first state as a distinct sjdc- 

 cies, which he named Purpura imbricata. The English 

 shells of the genus Pinna (and doubtless the foreign ones 

 also) offer the same variations, which have given rise to 

 similar subdivisions of species. Shells which have branch- 

 ing or expanded varices, like the Murices, are also much 

 influenced by these circumstances ; and hence many mere 

 varieties, arising from local causes, have been considered as 

 distinct species. Thus Murex angulifcr is merely a Murex 

 ramosus with simple varices ; and Murex crinaceus, M. toro- 

 sus, M. subcarinatus, M. cinguliferus, M. tarentinus and M. 



