Jackson.] 542 [April 4, 



young shell, and allow it to remain a short time, it can be readily 

 picked up on the point of a knife. Quite large spat may be re- 

 moved when dead (i. e., the organic matter inert) by soaking or 

 boiling. Treatment with acid, on the other hand, does not loosen 

 the hold of the shell. Quantities of young spat die during the 

 summer, yet few dead shells are found on cultch, because soon 

 after death the organic cement decays and is dissolved, and the 

 shell falls off. Dead oyster shells exposed to water or weather 

 sooner or later loosen their hold upon the stone or other object to 

 which they had attached themselves when young. Professor Hux- 

 ley (8) notes that the European o}^ster separates naturally from un- 

 favorable objects of support ; and that the separation takes place 

 naturally at an early stage can be seen markedly in many fossil 

 members of the family. Conversely, the calcareous plug of Ano- 

 mia, which appears to be fixed to the object of support by a purely 

 calcareous union, remains indefinitely after the death and separa- 

 tion of the individual to which it belonged. All our evidence is 

 therefore in favor of the assumption that the cement by which the 

 oyster is attached is wholly organic. 



With the introduction of the spat stage, a fundamental histolog- 

 ical difference arises abruptly in the structure of the shell. As we 

 have stated above, the prodissoconch consists of lime infiltrating an 

 amorphous matrix of conchyolin, but in the initial spat stage the 

 first layer of shell is deposited in a "tessellated or prismatic" manner 

 in a horny matrix, as described and figured by Professor Ryder (22, 

 24). Soon after the formation of the spat stage, the subnacreous, 

 white, porcellanous layer begins to be deposited on the yellowish- 

 brown, prismatic layer, first making its appearance as irregular 

 blotches in the centre of the valve. These two layers continue 

 throughout the rest of the life of the oyster, and together, though 

 in disproportionate degree, build up the massive adult shell. The 

 prismatic layer always remains thin, as may be readily seen by sec- 

 tions, or by treating an adult with acid, when the remaining con- 

 chyolin of the prismatic layer will separate from the overlying 

 subnacreous layer like a dissected skin. 



The protoconch of Owen, in Cephatopods, is the early shell 

 which precedes the conch, or true shell. Professors Hyatt (13) 

 and Brooks (3), consider the protoconch in cephalous molluscs as 

 the univalve shell of the fully developed veliger, and probably de- 



