layers of shell described in this chapter are those 

 which are found in more recent biological publi- 

 cations (Korringa, 1951; Leenhardt, 1926). 



The shell of the oyster consists of four distinct 

 layers: periostracum, prismatic laj-er, calcite- 

 ostracum, and hypostracum. The periostracum 

 is a film of organic material (scleroprotein called 

 conchiolin), secreted by the cells located near the 

 very edge of the mantle. The periostracum is 

 very poorly developed in C. virginica and cannot 

 be found in old shells. It covers the prismatic 

 layer which can be best studied by removing from 

 the edge of an oyster a small piece of newly formed 

 shell. Microscopic examination reveals that the 

 prismatic layer is made of single units shown in 

 figure 44. Each prism consists of an aggregate of 

 calcite crystals (Schmidt, 1931) laid in a matrix 

 of conchiolin which after the dissolution of mineral 

 constituents in weak hydrocliloric acid retains the 

 general configuration of the prisms (fig. 45). The 

 double refraction of the walls of empty prisms is 

 pronounced and causes slight iridescence notice- 

 able under the microscope. In a well-formed 







Millimeters 



0.5 



Figure 44. — Prismatic layer at earlier stages of calcifica- 

 tion. C. virginica. 



MORPHOLOGY AND STRUCTURE OF SHELL 



layer the prisms are wedge-shaped and slightly 

 curved (fig. 46). Conchiolin adiiering to the 

 prisms can be destroyed by boiling in potassium 

 hydroxide solution and the prisms separated 

 (Schmidt, 1931). Their shape and size are very 

 variable. 



The optical axes of the prism are, in general, 

 perpendicular to the plane of the prismatic layer, 

 but in places they are irregularly inclined toward 

 it. 



Calcite-ostracum, called also a subnacreous 

 layer (Carpenter, 1844, 1847), makes up the major 

 part of the shell. The layer consists primarily of 

 foliated sheets of calcite laid between thin mem- 

 branes of conchiolin. The separate layers are 

 irregularly shaped with their optical axes in ac- 

 cidental position (B0ggild, 1930). In a polished, 

 transverse section of the shell of C. virginica the 

 folia are laid at various angles to the surface 

 (fig. 47). This layer is frequently interrupted by 

 soft and porous chalky deposits (upper two layers 

 of fig. 47) which appear to consist of amorphous 

 material. It can be shown, however, that chalky 

 deposit is formed by minute crystals of calcite 

 oriented at an angle to the foliated lamellae of the 

 hard material. 



Hypostracum is a layer of shell material under 

 the place of the attachment of the adductor muscle. 

 In the shells of C. virginica the layer is pigmented 

 and consists of aragonite (orthorhombic calcium 

 carbonate, CaCOs). 



For many j^ears oyster shells were considered to 

 be composed entirely of calcite (B0ggild). Re- 

 cently Stenzel (1963) has discovered that on each 

 valve of an adult f. virginica aragonite is present 

 as padding of the muscle scar, in the imprint of 

 Quenstedt's muscle, and in the ligament. 



As the oyster grows the adductor muscle in- 

 creases in size and shifts in the ventral direction. 

 The new areas of attachment become covered 

 with aragonite while the older, abandoned parts 

 are overlaid with the calcite. The progress of 

 the muscle from hinge toward the ventral side can 

 be clearly seen on a longitudinal section of the 

 shell where it can be easUy distinguished by its 

 darker color and greater hardness of the secreted 

 material (fig. 48). 



ORGANIC MATERIAL OF THE SHELL 



After the removal of mineral salts of the shell by 

 weak acids or by chelating agents, such as sodium 

 versenate, the insoluble residue appears in the 



37 



