282 ROBERT TRACY JACKSON ON THE 



condition in tlie prodissoconch. In Mya, Modiola, Vcnnis and most other genera stndied 

 (.section xiv) no prismatic layer was observed in the first stages of dissoconch growth 

 and this diflerence, with others considered, is deemed sufiicient to sepai-ate widely these 

 two divisions of genera. 



A discovery of mnch impoi-tancc is the existence of two adductor muscles in the pro- 

 dissoconch stage of Ostrea, PI. xxiv, figs. 1-2. From the form of tlie sliell and from 

 analogy, a similar condition is predicated in Pecten, Avicnla, Pcrna, Anomia, etc. All 

 these genera possess but one adductor muscle in the adult and it lias not been demon- 

 strated before (excepting in Ostrea in my preliminary paper) that two exist in the young. 

 Professor Huxley in 1883 showed that the first formed muscle of the oyster (fig. 25, 

 p. 300, this paper) could not be that of the adult and that a new, second muscle to be re- 

 tained throughout life must develop. On this basis and knowledge of early types of 

 Pelecypods, he argued that Ostrea must have developed from a diniyarian ancestry and 

 my discovery of the early two-muscled stage, therefore proves the truth of his assumption 

 (discussion, sections iv and xv, also brieffy in other sections on Perna, Pecten, etc.). 



There is commonly little consideration given to the true relations of the axes of the 

 body to the shell in desciibing Pelecypods. In dimyarians the hinge is dorsal and in 

 monomyarians the hinge area is also considered as dorsal almost universally. Professors 

 Hyatt (28), Brooks (9), Ryder (60) andLacaze-Duthiers (39), have pointed out the true 

 condition of affairs in Ostrea and Anomia, where on account of the changed position of 

 the axes of the body the hinge line is anterior, not dorsal. I have endeavored to main- 

 tain this more careful consideration of the true relation of parts in my paper and this 

 must be borne in mind in reading the text. For instance, in Pecten tlie hinge line being 

 anterior, the free ends of the valves are posterior and the ears are truly ventral and 

 dorsal, not anterior and posterior as usually described. This changed position of the 

 axes is intimately connected with the relative size of the adductor muscles of the valves; 

 a change in position coinciding serially with the deci-case and final loss of the anterior 

 adductor, together with the increase and final exclusive retention of the posterior ad- 

 ductor, as described in section v. 



The Ostread^e is a group having a peculiarly modified highly irregular shell which has 

 been a perplexing problem to naturalists in seeking the affinities of the group. As the 

 result of my studies I have come to the conclusion that the ostrean form of the shell is 

 due to the mechanical conditions of direct cemented fixation. The proof of the conclu- 

 sion is that all Pelecypods wiiich are directly attached by the cementation of one valve 

 assume a similar form of growth. The form claimed as the result of cementation of one 

 valve is a concave, highly modified, attached valve and a less modified, commonly much 

 flatter free valve, often differing so widely from the attached valve, as not to be recog- 

 nizable as belonging to the same species unless found in place. Cementation also appar- 

 ently induces a camerated structure of the shell. (See PI. xxv, fig. 8 and PI. xxvi, 

 figs. 1-15, as proof of the effects of cemented fixation.) This statement contains im- 

 2)ortant conclusions, which are discussed in the text (sections vii and xi) but they may 

 be briefly stated here. The normal Pelecypod is bilaterally equivalvular, but when at- 

 tached this feature is lost, asymmetry replacing the typical condition. The cemented 

 valve is highly modified, the free valve less markedly so; therefore it is the free valve 



