Jackson.] 540 [April 4, 



The anterior muscle has disappeared, and the posterior, which is 

 retained, lies in the middle lower portion of the shell, in a position 

 closely similar to that of an oyster. 1 



The revolution of the axes in Lamellibranchs, relatively to the axis 

 (hinge) of motion of the valves, is thus seen to be closely correlated 

 with the reduction and final loss of the anterior adductor, together 

 with the increase and final exclusive retention of the posterior. 



THE SHELL. 



The two valves of an adult oyster are together homologous with 

 the single valve of adult cephalous molluscs. Both originate from 

 the preconchylian gland, one as a single, the other as a paired 

 organ. The adult shell of cephalous mollusca is termed a conch. 

 Therefore, in view of its homology and double character, I suggest 

 the name dissoconch 2 for the shell of an adult oyster, and simi- 

 larly the same terminology is applicable to the adults of other 

 Lamellibranchs. 



The shell of oysters, at different periods of growth, presents strik- 

 ing dissimilarities. There are three well-marked stages of growth, 

 as pointed out and figured by Professor Ryder (21) : 



(1) Prodissoconch, symmetrical fry stage; (2) Silphologic 

 (spat) stage, flat left lower valve, and convex upper right valve; 

 (3) Adult, flat upper right valve and concave lower left valve. 



We will now consider the form and structure of the shell of the 

 oyster, commencing with the earliest shell and continuing through 

 the spat stages of growth. 



The early prodissoconch as figured by Ryder (22), figs. 1-2, and 

 others, is nearl} 7 discoidal, flattened into a straight line at the hinge 

 area, fig. 1, pi. iv. The valves are convex, equi valvular, with 

 concentric lines of growth, but farther apart ventrally, and closely 

 underlying one another dorsall} r , so that as growth continues the 

 centra are pushed forward, and lifted upward and outward until 

 the close of the stage. By this method of growth, relatively high 

 umbos are developed and the early straight hinge line is finally lost 



1 This case is particularly interesting, as the change takes place much later than in 

 the oyster. The two-muscled stage is not an embryonic one, but of a much older 

 period of development. In Aetheria, a near ally, the two muscles are retained through- 

 out life, in a partially revolved position. 



2 Aio-o-os, double; Koyx^, shell. 



