ROOM XII.] NATURAL HISTORY. 75 
mantle united, and which are peculiar for having the carti¬ 
lage of the hinge inserted in an internal cavity, as the 
genera Mactra and Lutraria , which are equivalve, and 
have cardinal teeth; Mya, Corbula, and Pandora , which 
are inequivalve ; and Anatina and Mygdala , which have a 
peculiar piece of shell placed on the side of their cartilage. 
The rest of the animals of the Bivalve shells have the 
lobes of the mantle separate from each other all the way 
round, and no syphons. Some of these animals have one 
very large adductor muscle near the centre of the shell, as 
the Oysters, which -are irregular and laminar, and the 
Pectens , and Hasps, which are regular, with a process 
called an ear, placed on each side of the hinge; most of 
these, in their young states, are attached by a beard, which 
passes out of the nick under the front ear of the right 
valve. Next follow the genera Hinnites, which is like the 
Pectens when young, but becomes attached and irregular 
in its adult state; and Anomia , which is peculiar for its 
shell being pearly, and having the right valve deeply 
nicked near the hinge* for the passage of a cartilaginous 
band by means of which it is attached to rocks and shells, 
its own form becoming gradually moulded to the surface it 
rests on. Thus, if the shell is found on a Pecten, it is 
ribbed, and if on the spine of an Echinus or the stem of a 
sea-weed, it is compressed and subcyiindrical. The other 
Bivalves have two subequal adductor muscles. 
Cases 14, 15, 16 contain those shells which are found 
in fresh water, as the Anodons and Uniones . They are 
peculiar for being pearly internally, and covered with a 
thick hard periostracum. They often yield pearls, which 
are caused by a disease that induces them to deposit the 
matter of which the inner coat is constructed, in a more or 
less globular form. The species of these genera vary ex¬ 
ceedingly in size, structure, and colour, according to the 
clearness, rapidity, or stillness of the water in wdiich they 
are found. Those that are found in ponds are large and 
bright-coloured, if the water is clear; and those that live 
in rapid rivers, are thick and dark, and often eroded at the 
beaks : the beaks of all are rugose and plaited when young. 
Cases 17 and 18 contain the Muscles, {Mytill,) Horse 
Muscles, ( Modiola , Pinna , and Avicula ,) the animals of 
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