CO^CHITEEA. 453 



of St. Sulpice, Paris. (Dillwyn.; Captain Cook states that 

 the animal of this species sometimes weighs 20 lbs. and is good 

 eating. * 



Fig. 252 shows the animal of Tridacna, as seen on removing 

 the left valve and part of the mantle within the pallial line. 



Distribution, 7 species. Indian Ocean, China Seas, Pacific. 



Fossil, T. media. Miocene, Poland (Pusch). Tridacna and 

 Eippopus are found in the raised coral-reefs of Torres Straits. 

 (Macgillivray.) 



Sub-gemis. Hippopus, Ijam.'dTck. H. maculatus, PI. XYIII., 

 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 hyssus. 



Fa:mily X. — Cahdiad^. 



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 valve; 

 ligament external, short and prominent ; pallial line simple or 

 slightly situated behind; muscular impressions sub-quadrate. 



A7iimal 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. 



Caedium, L. Cockle. 



Etymology, hardia, the heart. 



Synonym, Papjaidea, Sw. 



Types, C. costatum, PI. XIX., Fig. 1. C. 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 fringed ; foot long, cylindrical, sickle-shaped, heeled. 



The cockle ( 0. edide) frequents sandy bays, near low water ; a 

 small variety lives in the brackish waters of the river Thames, 

 as high as Gravesend ; it ranges to the Baltic, and is found in 

 the Black Sea and Caspian. C. rusticum extends from the Icy 

 Sea to the Mediterranean, Black Sea, Caspian, and Aral. On 

 the coast of Devon the large prickly cockle {C. acideatum) is 

 eaten. 



♦ " We stayed a long time in the lagoon (of Keeling Id,), examining the fields of 

 corsl and the gigantic clam-shells, into which if a man were to put his hand, he would 

 not, as long as the animal lived, be able to withdraw it." (Darwin's Journal, p. 460.) 



