MOLLUSCA. 73 
Tridacna, so that the two genera are now usually united. It may be 
remarked that the lunule of Hippopus is not entirely closed. 
Fam. 8. Cardiide. In this family the siphons are very short. In 
Cardium (C. echinatum, pl. 76, jig. 42) the foot is long, slender, and bent 
forwards at an angle in the middle, and with the aid of it the molluse can 
leap with considerable force. The species live buried in sand and mud. 
Cardium edule is eaten extensively in Europe. It is so abundant in the 
North Sea that shiploads are raked up and taken to Holland and other 
places to be burnt into lime, and a good locality will furnish 200 or 300 
cargoes. Hemicardium cardissa ( pl. 76, fig. 41). 
Isocardia I. cor., pl. 76, fig. 34) has a subglobular shell, with the promi- 
nent umbones curved or contorted. The species figured inhabits the Medi- 
terranean and the coast of Ireland. 
Fam. 4. Cycladide. This family includes certain freshwater genera 
which have a general resemblance to marine forms, as Cardium, Tellina, 
and Venus. They have generally a smooth olivaceous periostraca. 
Cyclas (pl. 76, fig. 43) is a genus of small subglobular shells from one 
eighth to one half an inch or more in size, inhabiting ponds and streams, 
sometimes upon the bottom and sometimes beneath it. The foot is long 
and tongue-shaped, the mouth closed posteriorily, and there are two 
retractile siphons. Besides the small diverging cardinal teeth, there are 
short anterior and posterior lamellar teeth. When kept in vessels of water, 
it is said that they can traverse the surface with the aid of the foot, the 
body being suspended beneath. 
Pisidium is allied to Cyclas, but it is more wedge-shaped, and there is 
but one siphon. P. dubwum, Say (Cyclas), Gould, Invert. of Mass. p. 75, 
fig. 56; P. abditum, Hald., 1841; Proc. Acad. N.8. i. 53, has a strap- 
shaped foot covered with vibrillee, and considerably longer than the shell ; 
and it can be extended, attached, and the body drawn up. The animal 
advances in this manner over the bottom of a smooth dish, being sometimes 
vertical and sometimes throwing itself upon one or the other side. The 
longer or wedge-shaped end of the shell goes foremost and is anterior; and 
although the foot is chiefly extended in this direction, it can be turned back- 
wards, and indeed its flexibility is such as to allow it to be extended in any 
direction. The shell is one eighth of an inch long, pale yellow, the lines of 
growth very fine, the cardinal teeth indistinct, and the lamellar teeth robust. 
Lives in iinet beneath the mud. The eee is scarcely protruded 
beyond the margin of the shell, as it is in most species of Cyclas. Imma- 
ture young are found within them, as in Cyclas. 
Cyrena is a genus containing much larger and heavier shells than Cyclas, 
and chiefly confined to the fresh. waters of warm climates. Cyrena 
carolinensis and Rangia cyrenoides (or Gnathodon cuneatus) are found in 
the southern United “States both recent and seis The latter has some 
affinities with Mactra. 
Megadesma is represented by a single species (rveclusa, Chemnitz; 
 puradoxa, Born; radiata, Lamarck), about three inches long, thick in 
texture, of a triangular form, green with violet rays, the inside white 
277 
