85 
rate 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 Rasps, 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 subcylindrical. 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 exceedingly in size, structure, and colour, 
according to the clearness, rapidity, or stillness of the 
water in which 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 pleated when young. 
Cases 17 and 18 contain the Muscles, Mytili , Horse 
Muscles, Modiola , Pinna, and Avicula, the animals 
of which are peculiar for the foot being small, and 
furnished with a tuft of fibres at its base in front, by 
which the animal fixes itself to rocks, Sic. This 
beard> 
ROOM XII. 
Nat. Hist. 
