IX CHEMICAL COMPOSITION OF THE SHELL 253 



When the shell exhibits a crystalline formation, the carbonate 

 of lime may take the form either of calcite or aragonite. The 

 calcite crystals are rhombohedral, optically uniaxal, and cleave 

 easily, while the aragonite cleave badly, belong to the rhombic 

 system, and are harder and denser, and optically biaxal. Both 

 classes of crystal may occur in the same shell. 



Two main views have been held with regard to the formation 

 and structure of the shell — (1) that of Bowerbank and Car- 

 penter, that the shell is an organic formation, growing by inter- 

 stitial deposit, in the same manner as the teeth and bones of the 

 higher animals ; (2) that of Reaumur, Eisig, and most modern 

 writers, that the shell is of the nature of an excretion, deposited 

 like a cuticle on the outside of the skin, being formed simply 

 of a number of calcareous particles held together by a kind of 

 'animal glue.' Leydig's view is that the shell of the Mono- 

 tocardia is a secretion of the epithelium, but that in the 

 Pulmonata it originates within the skin itself, and afterwards 

 becomes free." 



According to Carpenter, when a fragment of any recent 

 shell is decalcified by being placed in dilute acid, a definite ani- 

 mal basis remains, often so fine as to be no more than a mem- 

 branous film, but sometimes consisting of an aggregation of 

 ' cells ' with perfectly definite forms. He accordingly divides 

 all shell structure into cellular and membranous^ according to 

 the characteristics of the animal basis. Cellular structure is 

 comparatively rare ; it occurs most notably in Pinna^ where the 

 shell is composed of a vast multitude of tolerably regular hex- 

 agonal prisms (Fig. 162 B). Membranous structure comprises 

 all forms of shell which do not present a cellular tissue. Car- 

 penter held that the membrane itself was at one time a con- 

 stituent part of the mantle of the mollusc, the carbonate of 

 lime being secreted in minute ' cells ' on its surface, and after- 

 wards spreading over the subjacent membrane through the 

 bursting of the cells. 



The iridescence of nacreous shells is due to a peculiar line- 

 ation of their surface, which can be readily detected by a lens. 

 According to Brewster, the iridescence is due to the alterna- 

 tion of layers of granular carbonate of lime and of a very thin 

 organic membrane, the layers very slightly undulating. Car- 

 1 Arch. Naturgesch. xlii. p. 209. 



