PECTEN. 279 



believe it to he so, vary so greatly with age, that its former 

 division into two or more species can excite but little sur- 

 prise and less reprehension. In the young it is free and 

 regular, but when mature becomes attached and, from 

 adapting its growth to the circumstances of its abiding- 

 place, distorted. Consequently it is rather from the younger, 

 than from the more aged examples, that, contrary to the 

 ordinary rule, we must derive the characteristics of the 

 species. At the former stage the contour, which in the 

 adult is often orbicular, is manifestly broader than long, 

 and the hinder auricle is only about half the length of 

 the very large anterior one ; whereas in old shells, the 

 auricles, being dependent upon situation for their develope- 

 ment, are occasionally small, and sometimes, the growth 

 being checked in front but permitted behind, are of almost 

 equal dimensions. The colouring is very variable : at 

 times the exterior is entirely red or white, at times marbled 

 with patches of either tint ; frequently this painting lingers 

 upon the umbones, whilst the lower disk is either mottled 

 or marked with wavy patches of nut-brown, or brownish 

 liver-colour, on a whitish ground ; more usually, however, 

 the umbones do not differ in their painting from the rest of 

 the surface. That which peculiarly distinguishes the species 

 is its sculpture ; this, however, is wholly obsolete, save 

 on the free umbones, in the attached lower valve, which 

 moulds itself to the peculiarities of the rock or shell to 

 which it is affixed, and is consequently almost invariably 

 distorted and sinuous in appearance ; the form of the upper 

 valve assimilates itself likewise to that of the lower, and 

 exhibits corresponding gibbosities and indentations : the 

 portion actually attached is destitute of colour. Extremely 

 numerous costellse (we have counted as many as forty in 

 the young, and may say nearly twice that number for the 



