920 



MOLLUSCA AND BRACHIOPODA 



FIG. 693. Section of shell of Pinna, taken 

 transversely to the direction of its prism. 



to the hexagonal shape. These are arranged perpendicularly (or 

 nearly so) to the surface of the lamina of the shell ; so that its thick- 

 ness is formed by their length, and its two surfaces by their extremi- 

 ties. A more satisfactory view of these prisms is obtained by grinding 

 down a lamina until it possesses a high degree of transparence, the 



prisms being then seen (fig. 

 693) to be themselves com- 

 posed of a very homogeneous 

 substance, but to be sepa- 

 rated by definite and 

 strongly marked lines of 

 division. When such a 

 lamina is submitted to the 

 action of dilute acid, so as 

 to dissolve away the car- 

 bonate of lime, a tolerably 

 firm and consistent mem- 

 brane is left, which exhibits 

 the prismatic structure just 

 as perfectly as did the 

 original shell (fig. 694), its 



hexagonal divisions bearing a strong resemblance to the walls of 

 the cells of the pith or bark of a plant. By making a section of the 

 shell perpendicularly to its surface, we obtain a view of the prisms 

 cut in the direction of their length (fig. 695) ; these are frequently 

 seen to be marked by delicate transverse stria* (fig. 696) closely re- 

 sembling those observable 011 the prisms of the enamel of teeth, to 

 which this kind of shell-structure may be considered as bearing a 

 very close resemblance, except as regards the mineralising ingredient, 



If a similar section be de- 

 calcified by dilute acid, the 

 membranous residuum will 

 exhibit the same resem- 

 blance to the walls of pris- 

 matic cells viewed longitu- 

 dinally, and will be seen to 

 be more or less regularly 

 marked by the transverse 

 striae just alluded to. It 

 sometimes happens in re- 

 cent but still more com- 

 monly in fossil shells, that 

 the decay of the animal 

 membrane leaves the con- 

 tained prisms without any connecting medium ; as they are then 

 quite isolated, they can be readily detached one from another ; and 

 each one may be observed to be marked by the like striations, 

 which, when a sufficiently high magnifying power is used, are seen 

 to be minute grooves, apparently resulting from a thickening of the 

 intermediate wall in those situations. These appearances seem best 

 accounted for by supposing that each is lengthened by successive 



FIG. 694. Membranous basis of the same. 



