290 MANUAL OF THE MOLLUSCA. 



pedal opening, large, anterior ; siphonal orifices surrounded by a thickened 

 pallial border ; branchial plain ; anal remote, with a tubular valve : shell- 

 muscle single, large and round, with a smaller pedal muscle close to it behind ; 

 foot finger-like, with a byssal groove ; gills 2 on each side, narrow, strongly 

 plaited, the outer pair composed of a single lamina, the inner thick, with 

 margins conspicuously grooved ; palpi very slender, pointed. 



The shell of Tridacna is extremely hard, being calcified until almost 

 every trace of organic structure is obliterated. (Carpenter.} 



TRIDACNA, Bruguiere. Clam-shell. 



Etym. Tri- three, dakno, to bite ; a kind of oyster. (Pliny.) 



Ex. T. squamosa, PL XVIII. fig. 15. 



Shell massive, trigonal, ornamented with radiating ribs and imbricating 

 foliations ; margins deeply indented ; byssal sinus in each valve large, close 

 to the umbo in front; hinge teeth 1.1, posterior laterals 2.1. 



A pair of valves of T. gig as, weighing upwards of SOOlbs. and measuring 

 above 2 feet across, are used as benitiers in the Church of St. Sulpice, Paris. 

 (Dillwyn.) Capt. Cook states that the animal of this species sometimes 

 weighs 20 Ibs. and is good eating.* 



Distr. 6 sp. 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, PI. XVIII. fig. 16. 

 The "bear's-paw clam" has close valves with 2 hinge-teeth in each. It is 

 found on the reefs in the Coral Sea. The animal spins a small byssus. 



FAMILY X. 



Shell regular, equivalve, free, cordate, ornamented with radiating ribs ; 

 posterior slope sculptured differently from the front and sides ; cardinal teeth 

 2, laterals 1.1 in each valve ; ligament external, short and prominent ; pallia] 

 line simple or slightly sinuated behind; muscular impressions sub-quadrate. 



Animal with mantle open in front ; siphons usually very short, cirrated 

 externally ; gills 2 on each side, thick, united posteriorly ; palpi narrow and 

 pointed ; foot large, sickle-shaped. 



CARDIUM, L. Cockle. 



Etym. Kardia, the heart. Syn. Papyridea, Sw. 

 Types, C. costatum, PI. XIX. fig. 1. C. lyratum, fig. 2. 

 Shell ventricose, close or gaping posteriorly ; umbones prominent, sub- 

 central ; margins crenulated ; pallial line more or less sinuated. 



* " We staid a long time in the lagoon (of Keeling Id.), examining the fields oi 

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

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



