386 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



remind one of the lateral hinge processes in Pecten or the scallop. The 

 hinge border of the spat is at first somewhat bent upwards, as may be 

 seen in Fig. 9, which is a side view of Fig. 8. This is also shown in 

 Figs. 5, 6 and 7, and isin conformity with the slightly upward inclination 

 of the hinge end of the fry shell. It results from this thatthe hinge border 

 of the newly fixed fry is at first free. To sum up, we find that the fry of 

 the oyster is at first permanently fixed by a pallial secretion, probably 

 conchioline, at the border of its valves, with the hinge end inclined up- 

 wards, and the free border of the lower valve in fixed contact with some 

 foreign object, as shown in the annexed figures, and that the hinge end 

 of the spat is free for some time, the whole of the rest of the under sur- 

 face of the lower valve being cemented fast with conchioline. This 

 marginal adhesion of the lower valves is well shown in Figs. 5, 6, and 

 7. The material from which I have obtained my figures was mainly ob- 

 tained in August last, adherent to old oysters in the vicinity of Saint 

 Jerome's Creek, Maryland. Most of the specimens, even the oldest, 

 which I have figured, had probably not been attached quite a week ; in 

 fact, some of them had evidently only just begun to develop the spat 

 shell. Fig. 2 is taken from a preparation of the brood Ostrea edulis, 

 which was presented to me by Mr. W. H. Walmsley. 

 U. S. Fish Commission, December 8, 1882. 



EXPLANATION OF PLATE. 



FlG. 1. Young American oyster, two days old, adherent to side of the McDonald ap- 

 paratus, June 24, 1882. Viewed from the side, showing the projecting velum v and 

 maDtle border m magnified 183 times. 



Fig. 2. Four shells of the fry of Ostrta ednlis, showing their form and variable size 

 when taken from the beard of the parent. Magnified 96 times. 



Fig. 3. Young American oyster on the eve of becoming converted into the spat, 

 having just become firmly attached during the last larval or umbo stage. Magni- 

 fied 96 times. 



Fig. 4. Young spat of the American oyster shortly after its transformation into the 

 spat, viewed as a transparent object, showing the fry shells iu the umbo stage and 

 the prismatic structure of the spat shell growing from the edges of the former. The 

 upper and lower valves are shown slightly displaced. Magnified 96 times. 



Fig. 5. Young spat of the American oyster showing the manner of the attachment of 

 its lower valve with the downwardly bent lateral ahe of the hiuge and margin of 

 the lower valve. Magnified 35 times. 



Fig. 6. Youug spat of the American oyster a little older than the preceding, magni- 

 fied to the same extent. 



Fig. 7. Lower valve of auother specimen of the young spat of the American oyster, to 

 show the great convexity of the lower valve of the fry shell and its abrupt transi- 

 tion into that of the epat shell. Like Figs. 5 and 6 this specimen was drawn insitu 

 from the old shell to which it was attached. Magnified 35 times. 



Fig. 8. Much older spat of American oyster detached and viewed from the lower side- 

 to show the groove of the margiu of the fry shell where that of the spat begins to- 

 be formed. Magnified 35 times. 



Fig. 9. The preceding viewed from the edge to show the depressed form of the spat 

 shell, its flat under valve, the inclined hinge border, the inclined and convex fry 

 shells and slightly convex upper valve of the spat. Magnified 35 times. 



