73 
ROOM XII., XIII.] NATURAL HISTORY. 
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. 
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 fur¬ 
nished with a tuft of fibres at its base in front, by which 
the animal fixes itself to rocks, &c. This beard, as it is 
called in the last genus, issues out of a groove in the front 
of the right valve. The Pinnae, like the Pond Muscles, 
differ according to the place they inhabit. Those which 
live in smooth water are thin and covered with rows of 
small scales, while those that are found in rough water are 
thick and rugose. 
Then follow the genera Area , Pectunculus and Nucula, 
the animals of which have a large foot divided at the end, 
and the hinge formed of many interlocking teeth. In Area 
the end of the foot forms a glutinous secretion, which at 
length hardens, by which the animals attach themselves to 
rocks, &c. and the shell is rhombic. In Pectunculus, the 
shell is orbicular, and the hinge line curved, while in Nucula, 
the hinge line is angular, with the cartilage placed in a 
pit at the angle. 
Lastly, follow the shells of the Branchiopodous Mol- 
lusca, which have two ciliated arms, one placed on each 
side of the mouth. Of these, only two genera are found 
in Britain, viz. Terebratula and Criopus; the first is 
affixed by a tendon passing out through a hole in the upper 
valve; the latter is attached by the outer surface of its 
under valve. 
THIRTEENTH ROOM. 
The upright glazed Cases round the room contain 
the general collection of Birds. 
Cases 1 to 12 contain the Raptorial or Birds of 
Prey, the Accipitres of Linnaeus, which constitute the first 
Order. They are characterized by strong feet with sharp 
claws, and a powerful bill, the latter covered at the base by 
a naked skin, or cere; their stomach is almost entirely mem¬ 
branous, and sternum broad, giving attachment to the 
muscles of their long wings. Some of them feed chiefly 
by day; their eyes are placed on the side of their head, 
E 
