59 



opod mollusks often in passing into the adult estate 

 cement their larval shell to the feip of the permanent one, 

 it occurred to me that these little cap-like structures on 

 the tips of the umbos of the young oysters, must in like 

 manner be regarded as the shell of the oyster larva at 

 the time it attaches itself to a foreign object. Upon 

 comparing the outlines of the supposed larval shells of 

 our oyster with those shown in a mounted specimen of 

 larval oysters taken from the mantle of the European 

 species, I found the likeness very striking but much 

 larger. These umbonal affixed larval shells measure on 

 the average 1-80 of an inch, which is probably the size 

 of the fry at the time of fixation and are represented in 

 position in Fig. 6 viewed from within, and Fig. 7 from 



Figures 6 and 7. 



without. Previous to attachment, the right and left 

 shells of both the American and the European species 

 are perfectly symmetrical, and it is not until after at- 

 tachment takes place that the valves show a disposition 

 to become unlike. These larval shells in the sj)at I 

 have examined are symmetrical, which is almost posi- 

 tive proof that they are really the temporary shelly cov- 

 ering of the fry when about to change its roving habit to 



