NATURAL HISTORY. 
137 
GALLERY.] 
hemispherical pieces of mother-of-pearl between the 
animal and the shell, which it eventually covers with 
a pearly coat. In other countries, spurious Pearls have 
been produced, for an equally laudable object, by placing 
pointed pieces of wire in a similar situation. 
These animals are divided into orders, according to the 
modification of the mantle and foot. 
I. In the more typical kinds, the hinder edge of the 
mantle is furnished within with two (separate or united) 
syphons, which are provided with a large fan-shaped mus¬ 
cle on each side, for the purpose of retracting them into the 
shell when the animal fears external danger. The edges of 
these muscles form, according to their extent, a more or 
less deep sinuosity in the hinder part of the muscular im¬ 
pression, which passes round the inner margin of the 
shell. They are also furnished with two distinct adductor 
muscles, for the purpose of closing the shell to protect 
the animal from injury ; these leave an anterior and 
posterior scar on the oblong elongate valves, which are 
generally provided with distinct cardinal teeth under the 
umbones. 
In the Phyllopoda, so called because the foot is gene¬ 
rally lamellar or elongate, the gills are not produced into 
the canal of the syphon ; the mantle-lobes are generally 
separate below, and the syphons are elongated, and ge¬ 
nerally separate from each other at the top. 
The family of Veneridce have short syphons, which are 
united together for the greater part of their length, and 
they have an external cartilage, and three diverging teeth 
in their hinge, and none, or only rudimentary lateral teeth. 
They live on the sea-shore- Some few, as Venerupis , form 
holes in rocks. 
The family of Cyrenidce are very like the former, but 
they are fluviatile, living in rivers and streams, and the 
shells have distinct lateral teeth, and are covered with a 
hard olive polished petiostraca. 
The family of Cockles ( Cardiadce ) have very short 
syphons, not leaving any syphonal inflections. Their 
foot is large and sickle-shaped, and the shell cordate 
and radiately ribbed, with two large teeth in each valve, 
placed in a cross, and distinct lateral teeth. 
The family of Mactridce have the cartilage separated 
