17 



Substanz "" of List (6) that the adductor muscle is attached 

 to the shell, and it is secreted by the modified mantle 

 epithelium of the muscle area, which in the adult is very 

 difficult to trace. 



The formation of lamellibranch shells is not yet 

 completely understood. The Intussusception theory of 

 Mery assumed that the shell was itself a g-rowing- body. 

 Reaumur, after Regeneration experiments, laid the 

 foundation of the Secretion theory, according to which 

 the shell is a secretion product of the animal. This is the 

 theory now generally accepted. The periostracum can be 

 traced to the actual secreting cells in the periostracal 

 groove of the mantle edge, but difficulties have arisen in 

 connection with the other layers, and tbere is no doubt 

 that the Intussusception theory originated through tbe 

 difficulty of understanding the formation of a complex 

 shell structure from a solution or secretion partly organic 

 and partly inorganic. In those Laraellibranchs where an 

 outer, prismatic, layer is present, tbis is secreted and 

 grows only at the mantle edge. The inner nacreous 

 layer, or that part of tbe lamellar layer of the Pecten shell 

 corresponding to it, is unlimited in growth, and is formed 

 by the outer surface cells of the mantle. 



The colour of the shell varies considerably. In 

 Pecten maximus the upper valve is very generally reddish 

 brown, tlie lower having a someAvhat lighter yellow tint; 

 both valves may be mottled with bands or streaks of burnt 

 umber or yellow. P. opercidaris varies still more, and 

 may be almost any shade of red, pink, orange, yellow, 

 purple or brown, with streaks and blotches. Botli species 

 are sometimes quite white, with a slight orange tint at 

 the umbos. The inner surfaces of the valves are smooth 

 and porcelain-like in appearance, with very frequently in 

 P. mn.rimusi a broad band of a dark chocolate or burnt 



B 



