430 MANUAL OF THE MOLLUSCA. 
The shell resembles Glycimeris in the shortness of its posterior 
side, and the extraordinary development of its epidermis; the 
animal most resembles Leda in the structure of its foot and 
ills. 
; Distribution, 4 species. United States, Canaries, West Africa 
(Gaboon River), Mediterranean, Australia, New Zealand. 
Burrowing in mud; 2 fathoms. 
Fossil, 4 species: Carb. —. Britain, Belgium. 
FAMILY V.—TRIGONIADA. 
Shell equivalve, close, trigonal, with the umbones directed 
posteriorly ; ligament external; interior nacreous; hinge-teeth 
few, diverging; pallial line simple. 
Animal with the mantle open; foot long and bent; gills two 
on each side, recumbent’; palpi simple. 
Triconia, Bruguiere (not Aublet). 
Etymology, Trigonos, three-angled. 
Synonym, Lyriodon, G. Sowerby. 
Example, T. Costata, Pl. XVII., Fig. 24. TT. pectinata, Fig. 
221. 
Shell thick, tuberculated, or ornamented with radiating or 
concentric ribs; posterior side 
angular; ligament smali and 
prominent; hinge-teeth 2.3, 
diverging, transversely stri- 
ated ; centre tooth of left valve 
“(7712 divided; pedal impressions in 
Sim front of the posterior adductor, 
and one in the umbo of the 
y left valve; anterior adductor 
imp apression close to the umbo. 
Animal with a long and 
pointed foot, bent sharply, heel 
prominent, sole bordered by two crenulated ridges; palpi small 
and pointed; gills ample, the outer smallest, united behind the 
body to each other and to the mantle. 
The shell of Trigonia is almost entirely nacreous, and usually 
wanting or metamorphic in limestone strata; casts of the in- 
Fig. 221. Trigonia pectinata.* 
* Fig. 221. From a specimen in alcohol; the gills slightly curled and contracted, 
they should terminate near the margin, between the arrows which indicate the inhalent 
and exhalent currents: a, a’, adductors; h/, ligament; ¢, t', dental sockets; 0, mouth, 
it, labial tentacles or palpi; p, pallial line; m,margin; f, foot; v, cloaca. 
