INTRODUCTION. 



5. Supposed section of a part of a 

 solid shell. 



externally by concentric strise, or lines of growth, while on 

 the inside, the edges of the laminae are consolidated into a 

 kind of enamel. If a perpendicular section of a solid portion 

 of a shell were magnified, it would present, in many instances, 

 an appearance resembling the diagram, fig. 5 ; a may be taken 

 to represent the horny part of 

 the layers which form the outer 

 coating, named " Periostaca," 

 or " Epidermis ;" the undula- 

 ting line b, is formed by the 

 edges of the calcareous layers, 

 and causes the striae, or lines of growth, which are often dis- 

 tinguishable on the surface of the shell ; the space c is the 

 middle part of those layers, and at d they are consolidated 

 into the enamel which lin^s the interior. 



In some species the layers are irregularly grouped together, 

 and their edges overlap €ach other, so that they are easily 

 separable, and advancing beyond each other, give a leafy 

 appearance to the external surface. This structure is termed 

 foliaceous. A very familiar instance of 'this may be observed 

 in the common oyster. If a specimen of this shell be broken, 

 the substance will be seen to exhibit a degree of looseness^ 

 and a magnifying glass will enable the student to trace 

 distinctly the laminae of which it is composed. The accom- 

 panying representation of a magnified section (fig. 6.) will 

 shew at a, the external sur- 



face, with the foliations or 

 leaves; at h, the parcels o^ 

 layers which form them; and 

 at c, the pearly structure 



produced by their COnsoli- 6. Section of an oyster shell enlarg'ed. 



dation, and by the subsequently deposited enamel which 

 covers their external surface. 



