40 CEYLON PEARL OYSTER REPORT. 



the ligament near the anterior end of the dorsal edge) is a sloping area of roughened 

 shell, marked with close-placed lines of growth. This area (Plate I., fig. 4) becomes 

 much more extensive, and less vertical in its slope, because of an increased thickening 

 of the hinge, in old shells, 'and its condition is a good guide to the age after the shell 

 has ceased to grow actively in length and breadth. 



The shell is composed of three layers, outer, middle, and inner. The very thin 

 outer layer is the uncalcified, cuticular " periostracum," an extremely delicate horny 

 layer which allows the colour of the layer below to show through, and which becomes 

 worn off in old shells. At the free margin of the shell the periostracum is very thin 

 and transparent, extends beyond the calcareous matter, and is reflected to join the 

 surface of the ectoderm cells of the mantle-edge in the longitudinal groove where it 

 is secreted. The periostracum is seen in several parts of the section represented in 

 Plate VIII., fig. 1, and also as a detached film in the groove on the mantle-edge in 

 Plate VIII., fig. 2, Per. ostr. 



The middle or "prismatic" layer of the shell shows what Cakpentek called, in 

 Pinna, a " cellular" structure being formed of calcai'eous prisms or columns running 

 transversely to the surface, and appearing as polygons in section (Plate VII., figs. 14 

 to 18, and Plate VIII., fig. 1). 



The carbonate of lime is laid down in an organic matrix of conchiolin, and is found 

 in the adult pearl oyster to be in the form of the mineral aragonite. The prismatic 

 layer is deposited by the mantle epithelium near the free edge, just behind the margin 

 which forms the periostracum ; and many such layers of prisms may be formed succes- 

 sively, each new one inside the last, as the shell grows. At the free edge of the shell 

 ami <>n the imbricating ridges these films may separate and stand out as in the section 

 shown mi Plate VIII., fig. 1. The red and brown coloration of the shell is in this 

 layer, certain of the prisms being charged with pigment, as shown in fig. 18 on 

 Plate VII. Various stages in the decalcification of the prisms is shown in fig. 19. 

 Complete decalcification reduces a section to a honey -comb-like network of conchiolin 

 (as shown in Plate VII., fig. 17), which is continuous with the very similar organic 

 periostracum lying over it. 



The inner layer is the " nacre," formed of numerous delicate lamellas of the organic 

 matrix conchiolin, and calcareous matter. It is transparent under the microscope, 

 allowing the " cellular " structure of the prismatic layer to show through it clearly 

 (see Plate VII., fig. 15), and is almost structureless, having merely a fine granular 

 appearance (Plate VII., fig. 21) injsurface view under a high power. The layers of 

 which it is formed show as a series of very closely placed contour lines (Plate VII., 

 fig. 20). The most conspicuous feature of the nacre is the beautiful iridescence, an 

 interference phenomenon due to the diffraction of light by the irregular free edges of 

 the numerous delicate lamellae, alternately calcareous and organic, of which the layer 

 is formed. The iridescence in the case of the Ceylon shell is singularly brilliant, hut 

 the nacre is too thin to be of much value in the arts. 



