MOLLUSCA. 63 
widely distributed in a recent and fossil state, and is not confined to any 
particular country or climate. 
These shells are free, and live upon a surface of mud or sand, from a 
trifling depth to twenty fathoms, and they are fished up as an article of food. 
They present several distinct forms, and many of them possess great beauty. 
Pecten pleuronectes is named after the generic name of the founder, 
because one side is dark colored, and the other white. P. jacobzeus was 
formerly worn by pilgrims who had visited the Holy Land. P. quinque- 
costatus occurs fossil in the cretaceous deposits of Europe and America, 
and there are numerous species belonging to the tertiary formations of the 
United States. Lima, a genus of free shells, is allied to Pecten. 
Spondylus (pl. 76, fig. 35) is a genus of attached, rough or spiny, and 
usually heavy and finely colored shells, allied to Pecten and Ostrea. There 
are two strong teeth in each valve, and a depression for the ligament. 
Hinnites is allied to Pecten and Spondylus, and possesses the peculiarity of 
being free until it attains a certain size, when it becomes permanently affixed. 
Fam. 5. Aviculide. This family includes the shells from which most of 
the pearls of commerce are obtained. They are allied to the two preceding 
families, and most of the genera are byssiferous, with pearly shells. The large 
well known shell (pl. 76, fig. 20), which sometimes attains a size of ten 
inches, produces the finest oriental pearls, as well as most of the mother-ot- 
pearl which is used in the arts. It forms the genus Weleagrina, Lamarck, 
although it is now considered not to be distinct from Avicula, and it is there- 
forenamed Avicula margaritifera, Linn. It inhabits the Indianseas. Avicula, 
according to the celebrated anatomist Poli, has the mantle unclosed, and 
fringed with tentacular appendages. The foot is small and secretes a byssus. 
The genus Malleus (J/. malleus, Linn., pl. 76, fig. 26) is remarkable for 
having the hinge margin extended in some species in the antero-posterior 
direction. The shell is very irregular, the foot secretes a byssus, and the 
mantle has a fringe of small tentacles. Perna (fig. 30) has an irregular 
shell, hinge straight, with a row of transverse furrows for the insertion of 
the hgament. The byssus passes through a gaping vacancy in the front 
of the shell. The genera Malleus, Perna, Vulsella, Crenatula, Catillus 
Inoceramus, and some others, are placed by some authors in a distinct 
family, dalled. 
Pearls are secreted upon the inside of the shell, or in folds of the mantle, 
the latter being the most regular; and as their quality depends upon that 
of the nacre, those shells which have this of a fine quality produce the best 
pearls. The pearls of common oysters are rough concretions of no 
commercial value, and similar concretions are sometimes formed by univalve 
species, the mantle of which has, of course, the power of secreting the 
calcareous matter of the shell. Although pearls are formed out of the same 
material as the shell, a bead turned out of the latter has not their peculiar 
lustre, because the arrangement of the material is different, the successive 
layers being plane in the shell and spherical in the pearl. On this account 
shaping an irregular pearl does not alter its lustre. Irregular pearls are 
sometimes worn without being shaped, when the form is agreeable. Pear- 
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