CONCHIFERA. 453 
of St. Sulpice, Paris. (Dillwyn.) Captain Cook states that 
the animal of this species sometimes weighs 20 lbs. and is good 
sating.* | 
Fig. 252 shows the animal of Tridacna, as seen on removing 
the left valve and part of the mantle within the pallial line. 
Distrisution, 7 species. Indian Ocean, China Seas, Pacific. 
Fossil, T. media. Miocene, Poland (Pusch). Tridacna and 
Hippopus are found in the raised coral-reefs of Torres Straits. 
(Macgillivray.) 
Sub-genus. Hippopus, Lamarck. H. maculatus, Pl. XVIII., 
Fig. 16. The ‘‘bear’s-paw clam” has close valves with two 
hinge-teeth in each. It is found on the reefs in the Coral Sea. 
The animal spins a small byssus. 
FAMILY X.—CARDIADA. 
Shell regular, equivalve, free, cordate, ornamented with 
radiating ribs; posterior slope sculptured differently from the 
front and sides; cardinal teeth two, laterals 1.1 in each yalvye; 
ligament external, short and prominent; pallial lune simple or 
shghtly situated behind; muscular impressions sub-quadrate. 
Animal with mantle open in front; siphons usually very 
short, cirrated externally; gills two on each side, thick, united 
posteriorly ; palpi narrow and pointed ; foot large, sickle-shaped. 
CarDiIum, L. Cockle. 
tymology, kardia, the heart. 
Synonym, Papyridea, Sw. 
Types, O. costatum, Pl. XTX., Fig.1. OC. lyratum, Fig. 2. 
Shell ventricose, close or gaping posteriorly ; umbones promi- 
nent, sub-central; margins crenulated; pallial line more or 
less sinuated. 
Animal with the mantle-margins plaited; siphons clothed 
with tentacular filaments anal orifice with a tubular valve; 
branchial frmged; foot long, cylindrical, sickle-shaped, heeled. 
The cockle (C. edule) frequents sandy bays, near low water; a 
small. variety lives in the brackish waters of the rivex Thames, 
as high as Gravesend; it ranges to the Baltic, and is found in 
the Black Sea and Caspian. OC. rusticwm extends from the Icy 
Sea to the Mediterranean, Black Sea, Caspian, and Aral. On 
the coast of Devon the large prickly cockle (C. aculeatum) is 
eaten, 
* “We stayed along time in the lagoon (of Keeling Id.), examining the fields cf 
pure: and the gigantic clam-shells, into which if aman were to put his hand, he would 
uot, as Jong as the animal lived, be able to withdraw it.” (Darwin’s Journal, p. 460.) 
