452 



MANUAL OF THE MOLLUSC A. 



Animal attached by a byssus, or free ; mantle-lobe extensively 

 united ; pedal opening large, anterior ; sipbonal orifices sur- 

 rounded by a thickened pallial border ; brancliial 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. (Car- 

 penter.) 



Tridagna, Bruguiere. Clam-shell. 



Etymology , tri, three, dahio, to bite ; a kind of oyster. 

 (Pliny.) 



Example, T. squamosa, PI. XYIII., Fig. 15. 



Shell massive, trigonal, ornamented with radiating ribs and 

 imbricating foliations • margins deeply indented ; byssal sinus 



Fig. 252. Tridacna Crocca, Lam. (Original.) 



a, the single adductor muscle ; ;;, pedal muscle, and pedal opening in mantle j 

 /, the small grooved foot ; b, byssus : t, labial tentacles ; g, gills ; /, the broad pallial 

 muscle ; between g and I is the renal organ ; m, the double mantle-margin ; s, the 

 siphonal border; i, inhalent orifice; e, valvular excurrent orifice. An. Nat. Hist. 

 1855, p. 190. 



in each valve large, close to the umbo in front ; hinge teeth 

 1.1, posterior laterals 2.1. 



A pair of valves of T. gigas, weighing upwards of 500 lbs. and 

 measuring about 2 feet across, are used as htnitiers in the Church 



