1888.] 541 packson. 



being superseded by one of a gentle curvature, fig. 5, pi. iv. At the 

 close of the prodissoeonch stage and before any spat growth has 

 taken place the valves are deeply concave and nearly equal. The 

 lower left valve is, however, somewhat larger and deeper than the 

 upper right valve as seen in fig. 6, pi. iv. The umbos are always 

 inclined upwards at about the same angle, and are directed dor- 

 sally as shown in above studies, fig. 2, pi. iv. They invariably 

 point to the left of the observer, viewing them from above, when 

 fixed to the object of attachment. 1 



The prodissoeonch is, as described by Ryder (22, 24), homoge- 

 neous and laminar in arrangement, "not prismatic" as in the im- 

 mediately succeeding stages of spat growth. It is composed of 

 lime infiltrating an amorphous matrix of conchyolin, as may be seen 

 by treatment with acid. One of the marked characters of the pro- 

 dissoeonch is the uniformity of shape and size found in different 

 individuals. It has not the ostrean tendency to variability which 

 is noticeable in later stages. 



The height of the fully developed prodissoeonch is about ^ of 

 an inch. It has already attained a size much greater than it had 

 when first attached, according to Ryder's (22) figure of a recently 

 attached individual. 



The left attached valve turned over and viewed from the lower 

 side is seen to be exactly like the right free valve in general ap- 

 pearance. It is not flattened nor are there other indications to 

 mark where it was attached, so that this attachment is evidently of 

 a very superficial and delicate nature. 



Ryder (22) figures an oyster just after it has set, which seems 

 very clearly to be effected by means of the mantle reflected over 

 the edge of the lower valve. There is, in the later prodissoeonch 

 and spat stages, an organic conchyolin attachment of the shell it- 

 self, thus described by Ryder (24) : "The cementing material 

 seems to be the organic matrix of the shell which forms a percep- 

 tible layer on the outside of the valves, and which constitutes the 

 epidermis or periostracum of the oyster." In the prodissoeonch, 

 as well as spat stage, the cement is so firm that, as Ryder says 

 (22), the shell may be broken before it can be removed. To prove 

 that this is an organic cement, if we put a drop of water on a dead, 



1 Professor Ryder informs me by letter that he has never seen an exception to this 

 nor have I, though I have sought for umbos pointing to the right. 



