SEA-FISHERIES LABORATORY. Bog 
muscle to a more central position. A shortening of the 
length of the body with a closer attachment of the viscera 
to the muscle (which plays a prominent part as a support, 
and rotates shehtly), would bring about the conditions 
observed. Throughout this Memoir, however, the hinge- 
line has been taken as marking the dorsal edge of the 
body. 
THE MUSCULATURE. 
Pecten belongs to the Monomyaria, since it possesses 
only a single adductor muscle. ‘The possession of one 
adductor muscle by certain lamellibranchs does not 
indicate genetic relationship, and species which are. Iso- 
myarian, Anisomyarian and Monomyarian may all be 
found in a single family. In addition to the adductor 
there are present, the orbicular retractor muscle of the 
mantle (pallial muscles), a single retractor muscle of the 
foot on the left side, the intrinsic muscles of the foot and 
visceral mass, and the heart or cardiac muscles. 
The Adductor Muscle of the Shell (fig. 1, A. s. and 
A. u. and fig. 47) is the posterior one of those forms with 
two adductors present. In the early stages, after the 
free swimming larva, we have first a protomonomyarian 
stage when the anterior adductor is formed and is alone 
present. The next is a dimyarian stage when the 
posterior adductor is present in addition to the anterior. 
These two stages are quickly passed through, the anterior 
adductor disappears and the posterior increases in size 
and takes up a more central position. This may be called 
the deutomonomyarian stage. The muscle stretches 
obliquely across the body from one valve to the other. 
The attachment to the shell is more dorsal on the right 
valve, and, owing to the fact that the fibres cross 
obliquely, the various organs of the body that surround 
