140 NATURAL HISTORY. [EAST. ZOOL. 
spinular periostraca ; the hinge is obscurely toothed ; the 
cartilages are internal, placed in a pit in each valve, and 
furnished with a peculiar shelly hinge-piece, which is either 
placed before it or sometimes so as to cover the whole 
of its surface. In most of the genera the shell is regular 
and free. In Auriscalpium , Periploma , Cochlodesma and 
Hemicyclostoma , the cartilages are in a pit on a projecting 
spoon-shaped process, and the hinge-piece is small. In 
Thracia the cartilages are only on a callosity of the hinge 
margin. In the Lyonsice and Myadorce , the cartilage pit is 
sunk into the hinge margin of each valve, and covered by a 
large flat hinge-piece ; the shell of the former is thin and 
of the latter thick, with very unequal valves, the left one 
being flat. Chamostrea and Myocliama differ from the 
rest in the shell being irregular and attached by the outer 
surface of one of the valves. 
The family of Myadce are very like the former, but 
the cartilage is placed on a spoon-shaped cavity in one of 
the valves, fitting into a pit in the surface of the other; 
there is no hinge piece, and the hinge is toothless ; the 
tubes are long and the lips small. 
The family of Corbulidce differ from the former in 
having small tubes and very large lips ; in the right valve 
being the largest; and in being provided with a large 
tooth in each valve in front of the cartilage pit. 
The family of Pandoridce have very unequal valves and 
pearly shells, with two diverging teeth in each valve, near 
die cartilage pit. They have short tubes, like the Corbu - 
lidce, but the foot is small, the gills on each t side are 
united into one, and it is the left valve that is the most 
concave. 
The family of Solenomyadce are very peculiar for being 
covered w r ith a very hard cartilaginous periostraca, which 
is much produced beyond the edge of the shell; they have 
only one gill on each side, no lips, and the foot truncated 
and ciliated round the edge of the radiated grooved end, 
by means of which they very quickly bury themselves in 
the mud. They have only a single large ciliated syphon. 
II. In the more aberrant orders the mantle is not pro¬ 
vided with any distinct syphons, but the lobes are either 
free behind or united together, and pierced with one or 
two holes for the passage of the water. 
