312 ROBERT TRACY JACKSON ON THE 



with a flat, lower left valve and convex, upper right valve; (tt) adult, flat, upper right 

 valve and concave, lower left valve. 



The early prodissoconch or phylembryonic stage of Ostrea edulis is equivalvular, nearly 

 discoidal and flattened into a straight line at the hiuge area. On the hinge line, minnte 

 interlocking teeth are visible when viewed from the dorsal side. Embryonic shells of 

 Ostrea edulis of this stage are represented in PI. xxiv, figs. 13-16. They were removed 

 from the gills of the parent and sent out as pi-eparations by a London dealer in micro- 

 scopical suj^plies. The age is similar to fig. 25, ]). 300. The valves are saucer-shaped, 

 and aie marked by concentric lines of growth closely approximated dorsally, but farther 

 apart ventrally and anterioi'ly, so that the centres of calcification arc already pushed pos- 

 teriorly by the growth that has taken place, as seen in figs. 13 and 15, viewed from the 

 left and right sides respectively. By polarized light the shell gives the characteristic 

 lines of crystallized carbonate of lime. A straight hinge line seems to be characteristic of 

 the phylembryonic stage of Pelecypoda before the development of umlios (see section 

 xv). As examples, I Avonld refer to embryos of Ostrea, PI. xxiv, figs. 13-16, Anodon, 

 Cai'dium, Modiolaria and Montacuta, PI. xxiu, figs. 7-11, also to Hatschek's figs. 19-21 of 

 developing Teredo and Ryder's (62) figure of an embryonic Ostrea mrginiana. 



The valves of the phylembryonic stage of Ostrea edulis, PI. xxiv, figs. 13-16, have 

 scarcely any umbos developed, whereas our next stage of shell development has promi- 

 nent nmbos. The completed prodissoconch of Ostrea virginiana, PI. xxiv, figs. 17-18, 

 has deeply concave valves which are nearly equal. The lower left valve is, however, 

 somewhat larger and deeper than the upper right valve, fig. 18. The umbos are highly 

 developed and are always inclined upward at a high angle, as the shell is invariably at- 

 tached by the extreme border of the left valve. The umbos are directed posteriori}^ as 

 shown in the studies of the soft parts, PL xxrv, figs. 1-2, and they invariably point to 

 the left of the observer viewing them from above, a necessary consequence of their be- 

 ing attached by the left valve.^ (For further discussion of the fixation of the oyster by 

 the left valve, see section iv.) Coincident Avith the development of high umbos, the 

 early straight hinge line, seen in the phylembryonic stage, is lost and is superseded by 

 one of a gentle curvature. The prodissoconch is the completed shell of the embryonic 

 free-swimming veliger period, and the young oyster becomes attached 

 permanently at the close of this period, the fixation consisting of the 

 soldei'ing of the ventral margin of the left valve to the object of sup- 

 port. 



The completed prodissoconch of Ostrea edulis, fig. 26, differs from 

 that of our species, according to Horst. The umbos are less devel- 

 FiQ.TiT Completed oped than in our species, but they ai-e directed posterioi-ly. Teeth, 

 prodissoconch of Oatrea ^Iso, are mentioned on the hinge line at this stage and earlier by both 

 iier of left valve in aa Horst and Lacaze-Duthiers, whereas in our species no teeth have 

 almost upright position been obscrvcd by either Ryder (63) or myself. The histological 

 (a er ors ). structurc of the shell, as described by Huxley and Horst, is similar 



to that of our species. Specimens of Ostrea edulis kindly sent to me by Dr. Ilorst show 

 lunnistakably the teeth and other features described. 



' I'rofessor Ryder informs me by letter that he has sought for umbos pointing to the right, 



never seen an exception to this nor have I, though I have 



