X 



_L 



Centimeters 



Figure 48. — Left valve of 0. (Alectryonia) mcgodon cut along the principal axis of growth. Ilypo.stracum (dark .striated 

 layer) forms a pronounced platform for the attachment of the adductor muscle, and can be traced to its original position 

 in the young oyster (right). Chalky deposits are regularly arranged between the layers of calcite. Also see fig. 41. 



scribed for calcite shells. Present knowledo;e of 

 the cliemistry of tlie org;anic constituents of tlie 

 sliell is inadequate. It seems reasonable to 

 assume that conchiolin like other proteins is not 

 a single chemical substance common to a large 

 number of firganisms, but that it differs specifi- 

 cally from animal to animal and may even vary 

 in the different parts of the same shell. 



The analysis of amino acids obtained by hy- 

 drolysis f)f conchiolin prepared from decalcified 

 shells showed (Roche, Ranson, and Eysseric- 

 Lafon, 1951) that there is a difference in the shells 

 of the two species of European oysters, O. edulis 

 and f. angulata (table 4). 



Table 4. — Amino acids from the conchiolin of two species 

 nf oysters 



[In part.s of 100 parts of protein according to Roche, Ranson. and Eysseric- 

 Lafon (1951)1 



Taking advantage of the fact that both calcite 

 and aragonite are present in the two distinct 

 layers of shell of the fan oyster (Pinna) and of the 

 pearl oyster (Findada) , the French investigators 

 (Roche, Ranson, and Eysseric-Jjafon, 1951) at- 

 tempted to determine whether there is a difference 

 in the chemical composition of the organic material 

 of the two layers of the shell of the same species. 

 They found that tyrosine and glycine occur in 

 higher concentrations in the prismatic layer than 

 in the nacreous part of shells. In the prismatic 

 layer of calcite portion the content of tyrosine 

 varies between 11.6 and 17.0 percent and that of 

 glycine between 25 and 36 percent. In the 

 nacreous part made of aragonite the concentration 

 of tyrosine was from 2.S to 6.0 percent and that 

 of glycine varied between 14.9 and 20.8 percent. 

 The significant differences in the contents of the 

 two amino acids in the two parts of the shell 

 may provide a clue for further studies of the role 

 of the organic component on the mineral form in 

 which the calcium carbonate is deposited by the 

 mantle. 



MUSCLE ATTACHMENT 



The place of attachment of the adductor muscle 

 or muscle scar is the most conspicuous area of the 



MORPHOLOGY AND STRUCTURE OF SHELL 



41 



