Lamarck’s Genera of Shells. 79 
internal; in either case it serves both to keep the valves toge- 
ther, and to open and shut them. If the ligament be external, 
it is extended when the shell is shut; and if the muscle, which 
keeps the valves closed, be relaxed, the mere elasticity of the 
ligament opens them. If, on the contrary, the ligament be in- 
ternal, it is compressed when the shell is shut, but on the 
muscle, which keeps it so, relaxing, the elasticity of the com- 
pressed ligament serves to open it. 
The conchifera have no internal shell; they are all aquatic, 
some living in fresh, others in salt, water. Most of them are 
free, some are fixed to marine bodies by their shell, and others 
by corneous filaments, to which the name of byssus has been 
appropriated, 
This class contains nineteen families, and is divided into two 
orders, viz..,Conchifera bimusculosa, and Conchifera unimuscu- 
losa. 
First Order. 
CoNnCHIFERA BIMUSCULOSA*. 
The shell presents internally two separate and lateral mus- 
cular impressions. 
This order contains thirteen genera, and is subdivided into 
four sections. 
Section Ist. 
CRASSIPEDAt. 
Mantle wholly or partly closed in front; foot thick, poste- 
rior ; shell, when shut gaping, at the sides. ‘This section con- 
tains four families. 
Ist Family. 
TusicoLartiaf, (contains 6 genera.) 
Shell either contained in a testaceous sheath, distinct from 
its valves, or incrusted wholly or partially in the sides of the 
sheath, or projecting beyond it. The conchifera of this family 
are borers, and bury themselves in stones, wood, and thick 
shells, but Some remain in the sands. 
* Having two muscles. + Crassus and pes, signifying thick-footed. 
} Tubus and colo, signifying inhabiting tubes, 
