452 



JIASTTJAL OF THE MOLLT73CA. 



AniTTial attached "by a bys3us, or free ; mantle-lobe extensively 

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

 rounded by a thickened paUial 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. (Car- 

 penter.) 



TuiDACNA, Bruguiere. Clam-shell. 



Etiimology, tri, three, dakno, to bite; a kind of oyster. 

 (Pliny.) 



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



Shell massive, trigonal, ornamented with radiating ribs and 

 imbricating foliations • margins deeply indented ; byssal sinus 



Fig. 252. Tridac7ia Crocea, Jj-axa.. (Original.) 



a, the single adductor muscle; p, pedal muscle, and pedal opening in ina"tle. 

 /, the small grooved foot; 6, byssus: t, labial tentacles; g, gills; ', the b^.a^ yrJlial 

 muscle ; between g and I is the renal organ ; m, the double mantle-msjgin ; ». the 

 eiphnnal 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 teelii 

 1.1, posterior laterals 2.1. 



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

 mojjGuring about 2 feet across, are used as benitiera in the Church 



