110 CLASS ACEPHALA. 
rocks is so thick and stout that the axe is required to sever 
it. The flesh, though tough, is edible. In 
HIPPOPUS 
The shell is closed and flattened before, as if truncated. 
(Chama Lazarus,Chemn.) In the 
CHama, Brug., 
Or the true chame, the shell is irregular, inequivalve, usually 
lamellar and rough, adhering to rocks, corals, &c., like that of 
an oyster. Its summits are frequently very salient, unequal, 
and curled up; the internal cavity frequently has the same 
form, without any external indication of the fact. The ani- 
mal (PSILOPUS, Poli) has a small foot, bent almost like that 
of a man; its tubes, if it have any, are short and disjointed, 
and the aperture in the mantle which transmits the foot is not 
much larger. Some species are found in the Mediterranean. 
There are also several that are fossil. 
DicERAS, Lam. 
Between diceras and the chame there is no essential diffe- 
rence, only the cardinal tooth of the former is very thick, and 
the spiral lines of the valves are sufficiently prominent to re- 
mind us of twohorns. Fossil shells from the Jurassic strata. 
(Dic. arietina, Lam.) In the 
IsocaRDIA, Lam., 
The shell free, regular, and convex, with spirally-curled sum- 
mits, divided anteriorly. The animal (GLossus, Poli) only 
differs from that of an ordinary chama in having a larger and 
more ova! foot, and because the anterior opening of its mantle 
begins to resume its ordinary proportions. 
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