SECT. XI. 



SHELLS Of MOLLUSC A. 



233 



are highly organized substances. Examined with the 

 microscope, they present remarkable varieties in some 

 of the natural groups of bivalve Mollusca ; the structure 

 of the Monomyarian Oyster is characteristic of the divi- 

 sion which has but one muscle ; the Pimyaria, having 

 two muscles, are represented by the Cockle. 



The exterior la- 

 minae at the edge 

 of the fragile valves 

 of a Pinna are often 

 so thin and trans- 

 parent that the or- 

 ganization of the 

 shells may be seen 

 with a low mag- 

 nifying power. A 

 fragment has the 

 appearance of a 

 honeycomb on both 

 surfaces (fig. 167), whereas its broken edge resembles 

 an assemblage of basaltic columns. The exterior layer 

 of the shell is thus com- 

 posed of a vast number 

 of nearly uniform prisms, 

 usually approaching to 

 the hexagonal structure, 

 whose lengths form the 

 thickness of the lamina, 

 their extremities its sur- 

 faces. When the calca- 

 reous part of the lamina 

 is dissolved by dilute 

 acid, a firm membrane Fig ' 168 ' Membr ^ n s a basis of the she11 of 

 is left, which exhibits a 



hexagonal structure (fig. 168), as in the original shell; 

 but it is only in the shells of a few families of bivalves 



Fig. 167. Section of shell of Pinna transversely to the 

 direction of its prisms. 



