1134 The American Naturalist. [December, 
There are four distinct periods in the development of the shell 
of Ostrea: I. The phylembryo‘ stage, with saucer-shaped valves, 
a straight hinge line, and but slightly developed umbos. II. The 
completed prodissoconch (Fig. 3), with a curved hinge line, well- 
developed umbos, nearly equal valves, and without prismatic 
cellular structure. In Ostrea edulis there are teeth similar to 
those of Nucula on the hinge line of the pro- 
dissoconch, but none exist in O. virginiana. 
III. The nepionic period (Fig. 4), with a flat 
attached and convex free valve. The triangular 
cartilage pit of Ostrea originates in the initial 
nepionic stages on the border of the prodisso- 
m. en Prodisso- conch valve (Fig. 5). The right valve of this 
virginiana; l, eft. period has a well-defined prismatic layer. IV. 
ee The adult, in which the attached valve is 
concave and the free valve is flat. 
In the Ostreide° the striking peculiarity of the shell is that 
the valves are unequal, and also dissimilar. This character is 
ascribed to the mechanical effects of the condition of cemented 
FIG. 4.—Ostrea virgini- FIG. 5.—Ostrea virginiana; tip of left 
ana; p, prodissoconch; s, valve; 2, prodissoconch ; s, spat or ike 
spat or nepionic growth. onic growth ; /, cartilage pit; 4, flange-like 
extension of shell over object of support. 
fixation acting on a Pelecypod shell. This conclusion is of great 
value in tracing the genetic relations of attached forms. Differ- 
ences in the two valves of attached forms may be seen in Spon- 
dylus, Hinnites, Mulleria, the Chamidze, Rudiste, etc. There 15 
abundant proof that the form is due to the condition of fixation, 
and not to gravity, as has been maintained. The form induced 
* A new term, designating an early embryonic stage, yet which is advanced enough to 
show the characteristics of the class or phylum to which the embryo belongs 
5 In my paper it is spelled Ostreadze. In deference to rule, as a family name it should 
have been spelled as above. See Proc. Am. Ass. Adv. Sc., Aug., 1877. 
