110 
NATURAL HISTORY. [NEW BUILDING. 
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 their 
habits, which are indicated by some having long syphons, 
to facilitate the admission of the aerated water and food, 
and others only a posterior slit between the lobes of the 
mantle, some having a foot for walking from place to 
place, for attaching themselves to other bodies, and some 
being quite destitute of this organ. 
I. In the more typical orders, 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 in>* 
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, w r hich 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. 
1. The family of Veneridce have short syphons, which 
are united 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. 
2. The family of Cyrenidce are very like, and have all 
the characters of the former, but they are fluviatile, living 
in rivers and streams, and the shells have distinct la¬ 
teral teeth, and are covered with a hard olive polished 
petiostraca. 
3. The family of Cockles ( Cardiadcz ) have very short 
syphons, not leaving any syphonal inflections on the 
shell. Their foot is large and sickle-shaped, and the 
