8 MOLLUSC A. 



pointed ends, in both valves, and found to enclose less 

 and less space, and to be farther and farther removed 

 from the largest and outer ones. (Fig. i6.) Then, 

 either with the free valve in hand, or upon the lower 

 valve, let them trace the edges of some of the shell- 

 layers all the way around, until they have satisfied 

 themselves that the two inner layers or those by which 

 they can see the oyster is enclosed, were the last ones 

 built by the animal. 



These observations can be made intensely interest- 

 ing, if, after this, they reverse the process, and trace 

 backwards again the lines of the ridges, until they 

 arrive at the smallest and youngest layers at the apices 

 of the valves, and realize that these represent the out- 

 lines of the valves which, although now widely sepa- 

 rated, were once united and enclosed the oyster when 

 it was a mere baby. 



Very often the larger oysters have younger and smaller 

 ones attached to them, which can be compared with the 

 apices of the valves in order to show that the latter unques- 

 tionably represent the younger stages of the shell. It is 

 practicable, also, in specimens properly roasted, to split a 

 valve into its constituent layers, and show the imprint of 

 the large muscle of the mantle margin, and the hinge at 

 diiferent stages of growth. Vertical sections through the 

 roasted shells of oysters do not show the general structure 

 very well ; but, when made through the living shell, they are 

 very instructive, and in fact necessary to a complete under- 

 standing of the structure. For example, to see the proof 

 of how the great muscle moves forward, such a prepara- 

 tion (Figs. 13, 14) should be made. Here we see the past 

 history of the muscle imprinted upon the face of the shell 

 it has travelled over, and learn that the scar is made as 



