STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 89 



Similar prominences and concretions — pearls which are not 

 pearly — are formed inside porcellanous shells ; these are as 

 variable in colour as the surfaces on which they are formed.* 



T\iQ fibrous shells consist of successive layers of prismatic cells 

 containing translucent carbonate of lime ; and the cells of each 

 successive layer coiTespond, 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 

 hke arragonite, or satin-spar. Horizontal sections exhibit a 

 cellular net-work, with here and there a dark cell, which is empty, 

 (fig. 21.) 



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 {pholadida) the carbonate of lime has an 

 atomic aiTangement like arragonite, which is considerably harder 

 than calcarious spar ; in other cases the difference in hardness 

 depends on the proportion of animal matter, and the manner in 

 which the layers are aggregated. f 



In many bivalve sheUs there occurs a minute tubular structure^ 

 which is very conspicuous in some sections of pinna and oyster- 

 sheU. 



The bracJiiopoda exhibit a characteristic structure by which 

 the smallest fragment of their shells may be determined; it 

 consists of elongated and curved cells, matted together, and 

 often perforated by circular holes, arranged in quincunx order 

 (fig. 32). 



But the most complex shell-structm*e is presented by the 

 porcellanous gasteropoda. These consist of three strata which 

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



* They are pink in turhinellus and strombus ; "white in ostrea ; white or 

 glassy, pm-ple or black in mytilus ; rose-coloured and translucent in "pinna. 

 {Gray) 



t The specific gravity of floating shells (such as argonanta and iantJdna) 

 is lower than that of any others. {Be la Beche.) 



