No. 522] 



THE CANADIAN OYSTER 



349 



the spat, being the vertical measurement from the top of 

 the umbo to the lowest level of the opposite edge when 

 the prodissoconch is placed in the position of a creeping 

 clam. 



The spat caught on glass exhibited the characteristic 

 color of the pelagic larva— the smallest varying towards 

 pink, the larger towards brown. Those taken on opaque 

 objects, on the other hand, presented a different appear- 

 ance—instead of having a pink, reddish or brown colora- 

 tion as one would expect from comparison with the larva, 

 or, instead of having a white appearance as might be 

 looked for by comparison with the older spat and adult 

 oysters, they preserved a shining, dark, metallic lustre 

 with a few faint radial lines. In the center of the dorsal 

 region could be distinctly recognized the larval shell 

 (prodissoconch) of the oldest free-swimming stage, pre- 

 senting a uniformity of appearance in all the specimens, 

 and measuring in the neighborhood of .369 X -384 mm. 

 in height and length. 



The spat-shell (young dissoconch) is deposited by the 

 thickened rim of the mantle in layers along the ventral 

 and terminal edges of the larval valves, but not to any 

 extent along the dorsal or hinge edge, which explains 

 the concentric lines below the umbos. The latest de- 

 posited parts around the margins are very thin and 

 delicate and exhibit a prismatic structure. At first the 

 shape varies little from that of the prodissoconch, but 

 soon the dissoconch becomes extended fore and aft of the 

 hinge-area in a manner that suggests the wings (ears) 

 of a scallop-shell, the lower parts preserving a pretty 

 uniformly curved outline. Later these alae cease to be 

 conspicuous and the whole outline may become irregular 

 and variable. Deep or shallow concentric creases pre- 

 serve more or less indication of stages of growth, and at 

 places may be portions of radial lines. The deeper con- 

 cavity of the left valve remains noticeable for a time 

 after fixation takes place, particularly in sections, but 

 a little later the lower valve seems to lag behind the 



