27 



iuusc](> to ;i mure eeutial position. A slioi'tenin<^ of the 

 leng'tli of the body with a closer attachment of the viscera 

 to the muscle (which plays a prominent part as a support, 

 and rotates slightly), 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 Mouomyaria, 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- 

 mj'arian, 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 miiscles), 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, .4. s. and 

 A. u. and hg. 47 j 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 j)resent 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 oa the right 

 valve, and, owing to the fact that the fibres cross 

 obliquely, the various organs of the body that surround 



