452 MANUAL OF THE MOLLUSCA. 
Animal attached by a byssus, or free ; mantle-lobe extensively 
united; pedal opening large, anterior; siphonal orifices sur- 
rounded by a thickened pallial border; branchial plain; anal 
romote, 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 T’ridacna is extremely hard, being calcified until 
almost every trace of organic structure is obliterated. (Car- 
perter. ) 
TRIDAONA, Bruguiére. Clam-shell. 
Etymology, tri, three, dakno, to bite; a kind of oyster. 
{Pliny.) 
Example, T. squamosa, Pl. XVIII., Fig. 15. 
Shell massive, trigonal, ornamented with radiating ribs and 
imbricating foliations: margins deeply indented; byssal sinus 
| 
\ 
a) 
YD) 
Wy Fa 
Ax i) 
AY 
yy 
by 
7 
<= 
NIRS G 
>" TiN : AKAN 
Regge 
2 
. 
7 ee\"* 
é 
Fig. 252. TZridacna Crocea, Lam. (Original.) 
a, the single adductor muscle; p, pedal muscle, and pedal opening in mavtle, 
f, the small grooved foot; 6, byssus: ¢, labial tentacles; g, gills; 7, the bvard pallial 
muscle; between g and 7 is the renal organ; m, the double mantle-margin; »s. the 
sipnoral border; 72, inhalent orifice; e, valvular excurrent orifice. An, Nat. Hist. 
1855, p. 190. 
in each yalve large, close to the umbo in front; hinge teetn 
1.1, posterior laterals 2.1. 
A pair of valves of 7. gigas, weighing upwards of 500 Iba. and 
nersuring about 2 feet across, are used as benitiers in the Church 
