262 GILBERT C. BOURNE. 



But here they have shared in the torsion of the other organs 

 of the anterior part of the body, have become twisted round 

 to the right_, have lost their primitive connection with the 

 right valv^e, and are inserted on the left valve only, serving 

 rather as accessory retractors of the byssus than as retractors 

 of the foot. It will be observed that both the anterior and 

 posterior retractors of the foot arise from double origins 

 above and below (morphologically right and left) of the 

 retractor muscle of the byssus, but that each is formed into 

 a single strand shortly before its attachment to the shell. 



The retractor muscle of the byssus is a large, coarsely- 

 fasciculated muscle, nearly circular in section, and passes 

 transversely from its attachment to the left valve to the 

 byssus cavity. It probably represents the right and left 

 retractors of the byssus of symmetrical Lamellibranchia, but 

 betrays no sign of its primitive paired origin. De Lacaze 

 Duthiers (8), however, considers that the right retractor 

 byssi muscle is aborted in Anomia, but it should be observed 

 (fig. 10) that the muscle is equally well developed above, that 

 is on the morphological right, and below, that is on the 

 morphological left of the byssus cavity. 



The byssus cavity is large and shallow, more or less oval 

 or lozenge shaped, and bordered above and below by muscular 

 lips, which are really the posterior continuations of the right 

 and left margins of the foot. The bottom of the cavity is 

 lined by a number of close-set, parallel folds or lamella) 

 running fore and aft in the direction of the long axis of the 

 shell. These lamellae form the byssus gland, the histology 

 of which will be described in the latter part of this paper. 

 But I may state here that the byssus of Enigma is not 

 calcified like that of Anomia, but consists of a number of 

 parallel plates of the byssus substance, secreted by the 

 epithelial cells covering the ridges of the byssus gland. 

 These lamella) are fused externally into a plate which is 

 directly and firmly fixed to the substratum to which the 

 animal is attached. 



The Gills. — The natural position of the gills, as seen 



