312 ROBERT TRACY JACKSON ON THE 



with a flat, lower left valve and convex, upper right valve; (4) adult, flat, upper right 

 valve and concave, lower left valve. 



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

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

 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 preparations by a London dealer in micro- 

 scopical supplies. The age is similar to fig. 25, p. 300. Tlie valves are saucer-shaped, 

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

 apart ventrally and antei'iorly, so that the centres of calcification are 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 umbos (see section 

 xv). As examples, I Avould refer to embr^'os of Ostrea, PI. xxiv, figs. 13-16, Anodon, 

 Cardium, Modiolaria and Montaeuta, PI. xxiii, figs. 7-11, also to Hatschek's figs. 19-21 of 

 develojiing Teredo and Rjaler's (62) figure of an embryonic Ostrea virginiana. 



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

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

 nent umbos. The completed prodissoconch of Ostrea virginiana, PL 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 arc 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 posteriorly as 

 shown in the studies of the soft parts, PI. xxiv, 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 with 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 

 soldering of the ventral margin of the left valve to the object of sup- 

 port. 



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

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

 FiG.^(i7 Completed o])ed than in our species, but they ai-e directed posteriorly. Teeth, 

 prodissoconch of o.itrea .^\gQ^ are mentioned on the hinge line at this stage and earlier by both 



eihilis, attached by bor- _ -r-v . i • i • • ,11 



der of left valve in an Horst and Liacaze-Duthiers, whereas ui our species no teeth have 

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

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



to that of our species. Specimens of Ostrea e(Z«?*s kindly sent to me by Dr. Horst show 

 unmistakably the teeth and other features described. 



' Professor Ryder informs me by letter that he has sought for umljos pointing to the right, 



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



