8 INTRODUCTION. 



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

 to represent the horny part of 0^^^^^**, 



the layers which form the outer , -<" '; ' 



coating, named " Periostaca" , - - J ..._„ ■■: ;C '' 



or "Epidermis;" the undula- *^=S^£^ — -" — - e/ $ 



ting line b, is formed by the 5 - Supposed section of a part of a 

 edges of the calcareous layers, solid shelL 



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 lines the interior,, 



In some species the layers are irregularly grouped together, 

 and their edges overlap each 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 accompanying re- 

 presentation of a magnified section (fig. 6) will shew at 

 a, the external surface, with ^zgg&^e^y^L 



the foliations or leaves ; at h, 'A' y /y% "3—% 



the parcels of layers which ;; V < , . ,^^ '' G* 

 form them; and at c, the ^£^'~ S ^^ s& -_ 



pearly structure produced by ^"" SSSa ~-= = S__ 8 r 



their consolidation, and by the 6 * Section of an °y ster shdl enlarged, 

 subsequently deposited enamel which covers their external 

 surface. 



CLASSIFICATION. 



The classification of shells, that is, their systematic arrange- 

 ment into classes, orders, families, genera and species, cannot 



