604 
BULLETIN OF THE BUREAU OF FISHERIES 
nation of the shells of small scallops, it appeared that the fully developed prodisso- 
conch shell was about 0.18 millimeter long and inequivalve with the left valve the 
larger as stated by Belding but differing from the adult, which has the right valve 
the larger. 
Accordingly inequivalve larvse were sought in plankton collections. Two such 
larvae were found. One of these was easily recognized as that of Ostrea virginica, 
the common oyster of commerce. The other, which proved to be the larva of the inter- 
esting oyster discovered at Beaufort (Gutsell, 1926) and identified as 0. equestris, 
attained a size much too large for the scallop. To this day, except for a few Anomia, 
only these two markedly inequivalve larvse have been found. 
Finally, late in the fall of 1927, attention was drawn to a larva with equal valves 
but with a shell outline suggestive of that of the prodissoconch to be seen at the 
umbos of postlarval scallops. This larva did not markedly exceed the size of such 
prodissoconch shells and, after careful comparison, was tentatively accepted as the 
larval scallop. Later there was received the “Report of Experimental Shellfish 
Station” (Wells, 1927) with two plates showing the larval development of the bay 
scallop. One of these plates (see fig. 22) consists of excellent photographs, one of 
which shows individuals with an early postlarval shell growth and beginning to 
assume the secondary straight hinge of the adult scallop. The photographs indicate, 
but do not conclusively demonstrate, an equivalve larval shell. In correspondence 
the author states that the larval shell is equivalve. The shell outline is that of the 
Beaufort form taken to be the scallop. 
Mr. Wells kindly furnished some material including larvse, like mine too poorly 
preserved to be helpful, and also some very early postlarvae. Examination of these 
latter revealed a curious asymmetry of the postlarval growth. (Fig. 2lj.) This 
tends to make the left prodissoconch valve appear the larger and probably is sufficient 
explanation of the semblance, with later dissoconchs, of prodissoconch asymmetry. 
It does not, however, explain Belding’s statement, based on examination of prodisso- 
conchs, that the left valve of the late scallop prodissoconch is the larger. 
In plankton collections, the equivalve larva assumed to be that of the scallop 
was taken during the scallop-spawning season and in the year when the “set” failed 
(1928) disappeared early in the spawning season. There seems little reason to 
doubt that it is the scallop larva. 
In a form which is to have the right valve the deeper, the temporary deepening 
of the left valve is a curious phenomenon. In this connection it is interesting to 
note that although all scallops normally rest on the right side, some, P. opercularis 
(Dakin, 1909), and our giant sea scallop of commerce have the left valve the deeper. 
The thought that the temporary deepening of the left valve of the postveliger shell 
is philogenetic therefore suggests itself. 
Belding figured the fully developed prodissoconch as without velum or anterior 
adductor, but with large (posterior) adductor, gills, large foot, rather complex ali- 
mentary canal, and an otocyst. He supposed that the velum disappeared as the foot 
was developed, so that by the time the foot was “perfectly developed” the velum 
had disappeared. 
My sketches of living specimens (see fig. 21-i) of the larva which I consider to be 
Pecten show both foot and velum present in specimens 0.18 millimeter long which is 
nearly as large as were obtained. What was taken to be the anterior adductor was noted 
in specimens at least up to 0.16 millimeter long, so that it seems probable that this struc- 
ture continues to the close of the larval (prodissoconch) stage. Gills were not noted. 
