SEA-FISHERIES LABORATORY. 349 
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 
Méry assumed that the shell was itself a growing body. 
Réaumur, 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 there is no doubt 
that the Intussusception theory originated through the 
difficulty of understanding the formation of a complex 
shell structure from a solution or secretion partly organic 
and partly inorganic. In those Lamellibranchs where an 
outer, prismatic, layer is present, this is secreted and 
grows only at the mantle edge. The inner nacreous 
layer, or that part of the 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, the lower having a somewhat lighter yellow tint; 
both valves may be mottled with bands or streaks of burnt 
umber or yellow. P. opercularis varies still more, and 
may be almost any shade of red, pink, orange, yellow, 
purple or brown, with streaks and blotches. Both 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. maximus a broad band of a dark chocolate or burnt 
Z 
