t 
180 LECTURE XL 
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 
calcarious 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 Liniicean 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 Gigai 
