116 The Official Guide to the 
or “Tooth Shell,” so called from the resemblance of the 
shell to the tusk of an elephant. There are some 
thirty species known, which are inhabitants of the 
temperate and warmer seas, being almost cosmopolitan 
in their distribution. 
The fourth class, or main division of the Mollusca, is 
known as PELECYPODA, from the form of the “‘foot,” 
which is more or less that of a hatchet. Its members 
are possessed of no true head or eyes ; do not wander 
in search of their food, although some kinds possess 
limited powers of locomotion by means of the foot, or 
are able to swim, propelled through the water by the 
opening and closing of their valves. The body is 
enclosed in two valves or shells hinged at the posterior 
margin; they are all aquatic, most of them marine, 
and many species burrow into the sand or mud; 
others attach themselves to rocks, and some even 
perforate stones, wood, or other substances, in which 
they take up their abode. . 
One of the best known bivalves is the Oyster, which 
needs no description. A remarkable form is AZa//eus 
vulgaris, the “‘ Hammer Shell,” so called from the 
peculiar shape the shell assumes with age. The Pearl 
Oyster is another interesting member of this family, 
some fine specimens of which will be noticed; it is 
known as A/eleagrina margaritifera. ‘The most impor- 
tant fisheries are in N. West Australia and Ceylon. A 
curious oyster, of the genus P/acuna, is known as the 
* Window Shell,” from the pearly translucence of its 
substance. ‘The “Fan Mussel,” or ‘‘Pinna,” is another 
remarkable form ; it is somewhat wedge-shaped, and 
attains a length of two feet. The Pinna is found 
embedded in the sand the pointed end downwards, 
where it moors itself by a long silky byssus. The 
“silk” of the byssus of the great Pinna is mixed with 
that of the silkworm and woven into gloves, etc., by the 
Itahans. 
