MOLLUSCA. 73 



Tridacna, so that the two genera are now usnallj united. It may l;e 

 remarked that the hmule of Ilippopus is not entirely closed. 



Fam. 3. CardiidcB. In this family the sij^hons are very short. In 

 Cardium {C. echinatxim^ pi. 76, fig. 42) the foot is long, slender, and bent 

 forwards at an angle in the middle, and with the aid of it the mollusc 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 

 ]!*rorth 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 {pi. 76, fig. 41). 



Isocardia {I. cor.., pi. 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. Cycladidce. This family includes certain freshwater genera 

 which have a general resemblance to marine forms, as Cardium, Tellina. 

 and Yenus. They have generally a smooth olivaceous periostraca. 



Cyclas {pi. 7Q,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 posteriorly, and tliere 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. dubium, Say {Cyclas), Gould, Invert, of Mass. p. 75, 

 fig. 56 ; P. abditiim, Hald., 1841 ; Proc. Acad. N. S. i. 53, has a strap- 

 shaped foot covered with vibrillfe, 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 chieflv 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 springs beneath the mud. The siphon 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 conflned 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 fossil. The latter has some 

 affinities with Mactra. 



Megadesma is represented by a single species {reclusa, Chemnitz; 

 paradoxa, Born ; radiata, Lamarck), about three inches long, thick in 

 'texture, of a triangular form, green with violet rays, the inside white 



277 



