BULLETIN OF THE UNITED STATES FISH COMMISSION. 385 



breaking the lower valve. The substance which effects this attach- 

 ment is without doubt the organic matrix of the shell, viz, the so-called 

 eonchioliue of the external horny covering, epidermis, or periostracum. 

 The lower valve of the spat when growing on a flat surface may con- 

 tinue to adhere throughout the whole extent of its under surface, until 

 it is nearly 2 inches in diameter, before its edge, together with that of 

 the upper valve begins to bend upwards and become free. This at- 

 tachment is effected very early, as I have met with it in spat a little 

 over an eightieth of an inch in diameter. When it is twice this size it 

 is scarcely possible to remove the young oyster entire from its attach- 

 ment without first breaking loose with it a little flake of the object upon 

 which it rests. When the lower valve of the fry shell is examined under 

 a microscope it is found that a faint groove runs around its border, be- 

 yond which it is abruptly continued into the shell of the spat. This 

 groove is perhaps more pronounced on the lower valve than on the 

 upper, and marks the point of transition from the very convex ventri- 

 cose valves of the fry to the depressed or flattened valves of the spat 

 or fixed stage of development. These facts indicate most conclusively 

 the means by which the final fixation is effected, viz, by cementing it- 

 self to some stationary object by means of a deposit of conchioline from 

 the mantle border upon which the animal continued to deposit layers 

 of calcic carbonate. This does not, it may be remarked, dispose of the 

 possible existence of a temporary or larval byssus and byssal gland, 

 which, by the way, no embryologist appears to have observed up to the 

 present time at least; but, as already remarked, our failure to find such 

 a structure with special adaptations of the microscope, in the apparently 

 attached fry in our aquaria now renders its existence somewhat doubt- 

 ful. We have already alluded to the fact that the mantle border of the 

 fry is deflected outwards over the edge of the lower valve before the 

 formation of any trace of the spat shell. This is shown in Fig. 1 in the 

 plate appeuded to this article. It may be that a byssal organ is de- 

 veloped to effect the first stage of adhesion prior to the deposition of 

 the horny cementing material which ministers to the permanent fixa- 

 tion of the spat. 



The further development of the spat shell is interesting in that the 

 deposit of lime is continued mainly from the free borders of the valves 

 and not from the hinge margin, as may be gathered from Figs. 4, 5, 6, 7, 

 8 and 9 of the accompanying plate. It will also be noticed that the 

 beak or umbo of the fry shell has a constant direction in all of the fier- 

 ures, showing that the dorsal and ventral margins of the animal, even 

 at this stage, are constant in position ; that the lower fry shell very 

 uniformly represents the rudiment of the left valve of the adult. At 

 the hinge margin the development of shelly matter is interrupted, as 

 shown by the figures. In fact, the valves of the spat are at first trun- 

 cated on a line with the hinge of the fry shell, and as the shelly deposit 

 is continued outwards alse are developed, as shown in the figures, which 

 Bull. U. S. F. C, 82 25 June 13, 1883. 



