112 CLASS ACEPHALA. 
Chemn. VI. xix. 194. Fawn-coloured or whitish, with 
twenty-six transversely-folded ribs. 
Under the name of HEMICARDIUM we might separate 
those species in which the valves are compressed from before, 
backwards, and strongly carinated in the middle ; for it seems 
almost certain that a modification of the animal must be a 
necessary consequence of this singular configuration. (Car- 
dium cardissa, VI. xiv. 143—146, &c.) 
DOoNAX, Lin. 
The donaces have very nearly the same kind of hinge as the 
cardia, but their shell is of a very different form, being a tri- 
angle, of which the obtuse angle is at the summit of the valves, 
and the base at their edge, and of which the shortest side is 
that of the ligament, or posterior side, a rare circumstance in 
this grade among bivalves. ‘They are generally small, and 
prettily striated from the summit to the edges. Their animal 
(PERONZA, Poli) is furnished with long tubes, which are 
received into a sinus of the mantle. Some of them are found 
upon the coast of France. (Donax rugosa, Chemn. VI. &c.) 
CycLas, Brug. 
Separated from venus by Bruguiéres. Like the cardia and 
donaces, has two teeth in the middle of the hinge, and before 
and behind two salient and sometimes crenulated plates; but 
the shell, as in several species of venus, is more or less 
rounded, equilateral, and transversely striated. The animal 
has moderate tubes; the external tint is usually grey or 
greenish. The cyclades inhabit fresh water. 
One species, the Tellina cornea, L., Chemn. VI. xiii. 133, 
is very common on the coast of France. M. de Lamarck sepa- 
rates the 
CYRENA, Lam., 
Where the shell is thick, slightly triangular and oblique, 
