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 Carpenter, 

 that the shell is an organic formation, growing by interstitial 

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

 animals; (2) that of E^aumur, 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 Monoto- 

 cardia 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 animal 

 basis remains, often so fine as to be no more than a membranous 

 film, but sometimes consisting of an aggregation of ' cells ' with 

 perfectly definite forms. He accordingly divides all shell struc- 

 ture into cellular and memhranous, according to the characteristics 

 of the animal basis. Cellular structiu-e is comparatively rare ; it 

 occurs most notably in Pinna, where the shell is composed of a 

 vast multitude of tolerably regular hexagonal prisms (Fig. 162 B). 

 Membranous structure comprises all forms of shell which do not 

 present a cellular tissue. Carpenter held that the membrane 

 itself was at one time a constituent part of the mantle of the 

 mollusc, the carbonate of lime being secreted in minute ' cells ' 

 on its surface, and afterwards spreading over the subjacent 

 membrane through the bursting of the cells. 



The iridescence of nacreous shells is due to a peculiar 

 lineation of their surface, which can be readily detected by a 

 lens. According to Brewster, the iridescence is due to the 

 alternation of layers of granular carbonate of lime and of a very 

 thin organic membrane, the layers very slightly undulating. 

 Carpenter, on the other hand, holds that it depends upon the 



^ Arch. Naturgesch. xlii. p. 209. 



