782 EEPOET OF COMMISSIONER OF FISH AND FISHERIES. [4J 



secretes slielly matter, which serves to cement the valve to its support.* 

 As the animal grows, the mantle deposits new layers of shell over its 

 whole surface, so that the larval shell valves become separated from 

 the mantle by the new layers, which crop out beyond their margins and 

 acquire the characteristic prismatic and nacreous structure. The sum- 

 mits of the outer faces of the umbones thus correspond with the places 

 of the larval valves, which soon cease to be discernible. After a time 

 the body becomes convex on the left side and flat on the right ; the 

 successively added new layers of shell mold themselves upon it ; and 

 the animal acquires the asymmetry characteristic of the adult." 



In my article entitled " On the fixation of the fry of the oyster," pub- 

 lished in the Bull. U. S. Fish Commission, II, 1882, p. 383-387, I have 

 already described the manner in which the young embryo of 0. vir- 

 (jinica affixes itself by the border of the mantle. Upon comparing the 

 above-quoted description, given by Professor Huxley, of the way in 

 which this takes place in that of 0. editlis, it will be observed that 

 there is little or no diiference in this respect between these two species. 

 I have, however, entered more fully into a description of the manner in 

 which the metamorphosis into the spat shell is effected than was done 

 by Huxley, having indicated in my Figs. 5, 6, 7, and 8 the pecteniform 

 or scallop-like appearance of the shell of the spat in its very young con- 

 dition, with the dorsally straight-bordered anterior and posterior alee of 

 the valves which are developed at this time. It is also a very signifi- 

 cant fact that the young oyster spat should resemble in its early con- 

 dition the form permanently assumed by some of its nearest allies, the 

 pectens. And it maj" be explained only by the well established doc- 

 trine that even highly specialized forms tend to reassume during the 

 early stages of their existence the form of the type from which they 

 have been evolved. 



The hinge border of the embryos of both 0. virginica and 0. eduUs 

 is straight, and in both species there is an umbo developed on both 

 valves of the larval shell during its later stages. This character is also 

 observed in the young stages of native oysters from other parts of the 

 world, as in those from the Pacific, on the coasts of California and Peru. 

 It is therefore very probably characteristic of all of the members of 

 the family. 



My observations upon the internal organization of the young spat 

 were made upon some that were removed from the smooth inner surface 

 of the dead oyster and clam shells, which had been sown on the bottom 

 in Cherrystone Eiver, Virginia, by Captain Hine and Mr. W. H. Kim- 

 berley in the spring of 1881. A number were removed from such 

 situations without injury, so that I could study them under the micro- 



L* The young oyster is not cemented directly to its fixed basis by the calcareous- 

 substance of the shell, but by the brown cement substance which is quite apparent 

 on the outer surface of the valves. This layer answers . to the periostracuui of the^ 

 adult, and is lirobably what was really meant by thfe speaker. ] 



