18 THE SHELL. 



attempts to procure pearls in this manner have not been finan- 

 cially successful. 



Similar prominences and concretions — pearls which are not 

 pearly — are formed inside porcellaneous shells ; these are as 

 variable in color as the surfaces on which they are formed. 



The fibrous shells consist of successive la^^ers of prismatic 

 cells containing translucent carbonate of lime ; and the cells of 

 each successi've layer correspond, so that the shell, especially 

 when very thick (as in the fossil Inoceramus and Trichites), will 

 break up vertically into fragments, exhibiting on their edges a 

 structure like arragonite, or satin-spar. Horizontal sections 

 exhibit a cellular network, with here and there a dark cell, which 

 is empty (i, 3). 



The oyster has a laminated structure, owing to the irregular 

 accumulation of the cells in its successive layers, and breaks up 

 into horizontal plates. 



In the boring-shells (Pholadidfe) the carbonate of lime has an 

 atomic arrangement like arragonite, which is considerably harder 

 than calcai'eous spar; in other cases the difference in hardness 

 depends on the proportion of animal matter and the manner in 

 which the layers are aggregated. 



In many bivalve shells there occurs a minute tubular struc- 

 ture, which is very conspicuous in some sections of Pinna and 

 oyster-shell. This tubular structure is frequently occasioned by 

 the growth of a confervoid sponge, hence great care is required 

 in determining whether the perforations are an essential part of 

 the shell. 



The Brachiopoda exhibit a characteristic structure by which 

 the smallest fragment of their shells may be determined ; it con- 

 sists of elongated and curved cells matted together, and often 

 perforated by circular holes, arranged in quincunx order (i, 5). 

 But the most complex shell-structure is presented by the 

 porcellanous Gastropoda. These consist of three strata which 

 readily separate in fossil shells, on account of the removal of 

 their animal cement (i, 6, 1). Each of these three strata is 

 composed of very numerous vertical plates, like cards placed on 

 edge ; and the direction of the plates is sometimes transverse in 

 the central stratum, and lengthwise in the outer and inner (as in 

 CyprsQR, Cassis, Ampullaria, and Bulimus), or longitudinal in 

 the middle layer and transverse in the others (e. g. Conus, 

 Pyrula, Oliva, and Voluta). 



Each plate, too, is composed of a series of prismatic cells, 

 arranged obliquely (45^), and their direction being changed in 

 the successive plates, they cross each other at right-angles. 

 Tertiary fossils best exhibit this structure, either at their broken 

 edge, or in polished sections. 



The argonaut-shell and the bone of the cuttle-fish have a 



