PHYLOGENY OF THE PELECYPODA. 311 



(lulas, PI. xxiTT. Cif^;. 22, may be found piled on one another to the lieight of several inches, 

 each individual with its axes in the same vertical plane. These comparisons go to prove 

 wliat was claimed earlier, that calcareous fixation of the oystei-'s lower valve took place 

 when it became attached. It is almost certain that if any form of prehensile attachment 

 had existed for an appreciable time, we should at least occasionally find evidence of the 

 arrangement of the shells in the line of least resistance to cuirents. 



VI. OsTUEA. Devklopment of the shell. 



In the study of the soft parts it is shown that the shell of the oyster develops from a 

 pi-imitive structure, the preconchylian gland, which is found not only in other genera of 

 I^elecypoda, but in other classes of mollusca as well. According to Horst, a simple 

 unpaired-cuticular membrane, my fig. 22, p. 297, the product of the cells of the shell- 

 gland precedes the 2)aired calcareous shell. 



The two valves of an adult oyster are together homologous with the single valve of 

 adult Cephalons molluscs (see p. 281). In both, the shell originated from similar struct- 

 ures, one as a single and the other as a paired organ. The adult shell of Cephalous mol- 

 lusca is termed a conch. Therefore, in view of its homology and strikingly different 

 double character, I have suggested (36) the name dissoconch' for the shell of an adult 

 oyster and similarly the same terminology is applicable to the shell of other Pelecypoda. 



The protoconch of Professor Owen, in Cephalopods, figs. 1-8, p. 292 and PI. xxrir, 

 figs. 15-16, is the early cup-shaped shell which precedes the conch, or true shell. A spi- 

 rally twisted protoconch in Gasteropods,^ PI. xxiir, figs. 19, 21, 23, and a saddle-shaped 

 periconch (protoconch) in Scaphopods, PI. xxiii, figs. 13-14, may be also found as an 

 eml)ryonic shell preceding the couch or true shell of these groups (see section iir). It 

 will be seen from the study of Ostrea that the true shell, that which characterizes the 

 adult, originates with the introduction of the spat stage, PI. xxiv, figs. 19-21 and PI. 

 XXV, figs. 1-7. The earlier shell, PI. xxiv, figs. 17-18, is strikingly different. It is the 

 completed shell of the embryonic stages, and as shown in previous studies, it has devel- 

 oped from structures closely similar to those found in Cephalous molluscs. The embryonic 

 shell of Ostrea, therefore, is distinctly the homologue of the protoconch and periconch. 

 In the oyster, however, this shell is not single- but double-valved and, therefore, deserves 

 a distinct name. As it precedes the dissoconch or true shell, I have suggested (36) the 

 name prodissoconch^ or early, double shell, for the fii'st-formed shell of the oyster. A 

 prodissoconch or embryonic shell, strikingly different from the succeeding dissoconch, is 

 figured in this paper in many widely separate genera of Pelecypoda (see Pis. x:xiv to 

 XXX and cuts in the text) and it may be safely considered as characteristic of the class. 



The shell of oysters, at different periods of growth, presents striking dissimilarities. 

 There are four well-marked stages of growth: (1) phylembryo, early embryonic stage; 

 equivalvular, with a straight hinge line; (2) completed prodissoconch, symmetrical fry 

 stage, with a curved hinge line and high umbos developed; (3) nepionic (spat) stage, 



'Jtrt-ffo?, double; hdy^rj, shell. ' //oo, before. 



' See footnote, p. 292, and discussions in section on 

 Classification of Stages of Growtli anil Decline. 



