KEY TO PLATE II 



P'igs. 1-6. Oyster spats at different short intervals after fixation, for 

 the sake of space reduced to one-third the magnification of fig. 22 of PL I., 

 now magnified 50 diameters. 



The larval shell (prodissoconch) maintains the same size as long as 

 it can be recognized in a spat. Larval shells of all spats maintain approx- 

 imately the same size, no matter how large the spats grow. In all the 

 figures the prodissoconch is shaded, the dissoconch only outlined. 



The spats soon become so large that a lower power objective has to 

 be used in order to see the whole spat at once. 



Fig. 1. Spat with a very thin rim of new (spat) shell. 



Oc. 5, obj. 4 = 55 X 6-9= -379 mm. 



Oc. 5, obj. 2 = 24 X 15-38= -369 mm. 



•369 X 50=18-5 mm. = approximately the length of the larval shell 

 as it appears in the figures. 



Fig. 2. Spat -4 mm. long. 



Fig. 3. Spat -5 mm. long. 



Fig. 4. Spat -75 mm. long. 



Fig. 5. Spat 1 mm. long. 



Fig. 6. Spat 1-25 mm. long. 



Fig. 7. Eight spats superposed so that the larval shells coincide — 

 the same effect as a single spat drawn at different stages in its growth 

 from -5 to 2-25 mm. 



