72 NATURAL HISTORY. [uPPER FLOOR. 
plates. The Linnean conchologists considered the Teredo 
as a univalve;, having mistaken the tubes for the shell of the 
animal ; and the Pholas they called a multi valve shell. 
Case 12 contains the genera which have the-lobes of 
the mantle united, and 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; M^a, Corhula, and Pandora, which 
are inequivalve ; and Anatma 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 state, are attached by a beard, which 
passes out of the notch 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 
notched 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 subcylindrical. The other 
Bivalves have two subequal adductor muscles. 
Cases 14, 15, 16 contain those shells which are found 
in fresh water, as the Unios. They are peculiar for 
being pearly internally, and covered with a thick hard pe- 
riostracum. 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 exceedingly in 
size, structure, and colour, according to the clearness, ra¬ 
pidity, or stillness of the water in Avhich they are found. 
Tliose that are found in ponds are large and bright-coloured, 
if the water be 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. 
