SHELLS OP MOLLUSCA. 



169 



animal ; but it also projects beyond it, so as to enlarge the capacity of 

 the shell ; and as the separation of the valves affords free exit to those 

 parts of the animal, which are capable of being projected beyond the 

 shell, there is obviously no need of any other provision- to maintain the 

 shell in its natural form. Thus in the shells of the Mollusca, increase 

 takes place at the surfaces and edges only. 



281 . The proportion of organic and calcareous matter in Shell differs 

 considerably in the various tribes. The former is sometimes present in 

 such small amount, that it can scarcely be detected ; and the condition 

 of the calcareous matter then obviously approaches that of a crystalline 

 deposit. But in other instances, the animal basis is very obvious; 

 remaining as a thick consistent membrane, after all the calcareous mat- 

 ter has been dissolved away by an acid. This membrane is formed of 

 an aggregation of cells arranged with great regularity (Fig. 43, a) ; the 

 cavities of which are filled with carbonate of lime in a crystalline state. 



Fig. 43. 





1 



Prismatic cellular structure of shell of Pinna: a, surface of lamina; fc, vertical section. 



ie form of the cells approaches the hexagonal ; their diameter varies 

 in different shells from l-100th to l-2800th of an inch ; their thickness 

 also is extremely variable, even in different parts of the same shell. 

 Thus we sometimes meet with a lamina of such tenuity, as not to mea- 

 sure l-100th of an inch in thickness, whilst in other instances, a single 

 layer may have a thickness of half an inch, or even (in certain large 

 fossil species) of an inch or more. In this case, the cells, instead of 

 being thin flat scales like the tessellated-epithelium ( 233), are long 

 prisms, somewhat like the cells of the cylinder-epithelium (Fig. 26), 

 with their walls flattened against each other. The appearance which is 

 then presented by a vertical section of them, is represented in Fig. 43, b. 

 The long prismatic cells are there seen to be marked by delicate trans- 

 verse striae, and these, taken in connexion with other indications, appear 

 to show, that every such prism is in reality formed by the coalescence 

 of a pile of flat cells, resembling those which are seen in the very thin 

 laminoe just described ; so that the thickness of the layer depends upon 

 the number of the cellular laminae which have coalesced to form its 

 component prisms. This character is of interest, as representing on a 

 magnified scale a corresponding appearance in the Enamel of human 



