180 LECTURE Xf. 



rations. In this case, the animal, conscious of the 

 weakness or deficiency of the shell in those spots, 

 soon begins to secure the weakened parts by de- 

 positing over them a great quantity of its pearly 

 catcarious matter, and thus forms so many pearly 

 tubercles over them. The practice however is, I 

 believe, considered as not of importance sufficient 

 to make it an object of gain, but rather of mere cu- 

 riosity; the pearly tubercles thus obtained be- 

 ing of inferior beauty to those more naturally 

 produced. 



The Linnaean genera of Bivalve Shells are 

 somewhat less numerous than the Univalves, and 

 .are principally constituted from the different struc- 

 ture of the teeth or prominences belonging to the 

 hinge of the Shells. Among the most remarkable 

 genera are those of Spondylus and Chama ; in the 

 former of these, the chief species, which resemble 

 Oysters in shape, are of rich colours, and beset 

 with numerous and differently shaped spines and 

 processes, giving the whole shell a singularly cu- 

 rious aspect. In the genus Chama, many species 

 of which greatly resemble those of Spondylus, we 

 liave an example of by far the largest and heaviest 

 of the whole testaceous tribe ; the Chama Gigas 



