R^uBB] LIFE-HISTOEY OF THE OYSTEE. 329 



lias not yet been certainly determined for any one of the three speciea 

 of which the development has been studied. In the case of the Ameri- 

 can species, however, it has been found by the wi-iter that under favor- 

 able circumstances attachment of the fry probably takes place within 

 twenty-four hours after fertilization. This appears to be effected by the 

 border of the mantle of the fry, where it is deflected over the edge of 

 the undermost valve, as represented at m, in Fig. 1 of Plate LXXV. 

 The attachment is a close one to whatever surface the fry may fix itself, 

 and at most, if there is a byssus developed, it is extremely short. The 

 young, however, after attachment, continue to grow as larvae, and have 

 a very symmetrical shell, as shown, enlarged 96 times, in Fig. 3, in the 

 plate. When the valves of the fry have acquired umbos the develop- 

 ment of the spat shell begins, as shown in Fig. 4, in the plate. The 

 spat shell is, however, different in its microscopical characters from that 

 of the fry, since we find that in the spat shell, or in that formed after 

 permanent fixation has been accomplished, the calcareous matter begins 

 to be deposited in a tesselated or prismatic manner. The transition is 

 a very abrupt one. The larval shell is homogeneous, but the calcareous 

 material of which it is composed is laminar in arrangement. As soon 

 as they are formed the beaks of the larval valves are invariably inclined 

 upwards in the spat, as may be seen in Figs. 6, 6, and 7, viewed from 

 above, and in Fig. 9, as seen from the side, in the plate. It is also a 

 fact that the beaks of the larval shells are invariably directed horizon- 

 tally in one way, as may be seen from the same series of figures. 



For a considerable time the whole under surface of the lower valve of 

 the spat remains flat and is cemented to the surface upon which it is 

 fixed by its under side. It is only after it has grown to the size of from 

 one-half to two inches across that its margin begins to bend upwards and 

 become free. The cementing material seems to be the organic matrix 

 of the shell which forms a perceptible layer on the outside of the valves, 

 and which constitutes the epidermis or periostracum of the adult. 



The history of the fixation of the common European oyster does not 

 seem to have been very well worked out, but it is probably not very dif- 

 ferent from that of our own species. The young of the former, however, 

 as taken from the beard, or mantle and gills of the parent, vary consid- 

 erably in size, as may be inferred from an inspection of Fig. 2, in the 

 plate, which represents four specimens enlarged 96 times, drawn to the 

 same scale, in different positions, and taken from a preparation in the 

 possession of the writer. 



In order to show the rate of the growth of the spat or affixed stage of 

 the oyster the following figures may be useful : That represented in Fig. 5, 

 Plate LXXVI, is supposed to be not over twenty days old; that in Fig. 

 6 is known not to be over forty -four days old ; that in Fig. 7, not over 

 forty -eight; Fig. 8, seventy-nine ; and Fig. 9, eighty-two days old. Fig. 

 10 represents an oyster of one summer's growth, collected from a wreck 

 at Cape May, N. J., by my friend, Mr. John Ford. The preceding figures, 



