MOLLUSCA. 73 



Tridacna, so that the two genera are now usually united. It may lie 

 remarked that the lunule of Ilippopus is not entirely closed. 



Fam. 3, Cardiidce. In this family the siphons are very short. In 

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

 forwards at an angle in the middle, and w'lth 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 

 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 {pi. 76, ßg. 41). 



Isocardia {I. cor.., pi. IQ^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. Cydadidcc. This family includes certain freshwater genera 

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

 and Venus. They have generali}'' a smooth olivaceous periostraca. 



Gyclas {pl.1Q.,fig. ^'S)'\% 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. diibium., Say {Cyclas)., Gould, Invert, of Mass. p. 75, 

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

 shaped foot covered with vibrillse, 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 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 confined to the fresh waters of warm climates. Cyrena 

 caroHnensis and Rangia cyrenoides (or Gnathodon cuneatus) are found in 

 the southern United States both recent and fossil. The latter has some 

 aflinities 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 



27V 



