ORDER TESTACEA. 91 
these threads as a secretion, spun and drawn from the furrow 
of the foot. Poli thinks that they are mere prolongations of 
tendinous fibres. 
The shell essentially consists of two pieces, called valves, to 
which, in certain genera, are added others, connected by a 
hinge that is sometimes simple, and sometimes composed of a 
greater or smaller number of teeth and plates, which are 
received into corresponding cavities. 
There is usually a projecting part near the hinge, called 
the summit, or nates. 
Most of these shells fit closely when the animal approxi- 
mates them, but there are several which exhibit a gaping por- 
tion either before or at the extremities. 
The first family of the testaceous acephala, or 
THE OSTRACEA, 
Have the mantle open, without tubes, or any particular aper- 
ture. 
The foot is either wanting in these mollusca, or is small ; 
they are for the most part fixed bythe shell or byssus to rocks 
and other submerged bodies. ‘Those which are free seldom 
move, except by acting on the water, by suddenly closing 
their valves. 
Ty the first subdivision, there is nothing but a muscular 
mass, reaching from one valve to the other, as seen by the 
single impression left upon the shell. 
It is thought proper to class with them certain fossil shells, 
the valves of which do not even appear to have been held 
together by a ligament, but which lay together like a vase and 
its cover, and were connected by muscles only. They form 
the genus 
