430 MANUAL OF THE MOLLTJSCA. 



The sliell 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 

 gill^. 



Distribution, 4 species. United States, Canaries, West Africa 

 (Gaboon Eiver), Mediterranean, Australia, New Zealand. 

 Burrowing in mud ; 2 fathoms. 



Fossil, 4 species. Carb. — . Britain, Belgium. 



Family V. — Teigo^ad^. 



Shtll 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. 



Tmgonia, Bruguiere (not Aublet). 



Etymology, Trigonos, three-angled. 

 Synonym, Lyriodon, G. Sowerby. 



Example, T. Costata, PI. XYII., Fig. 24. T. pectinata, Fig. 

 221. 



Shell thick, tuberculated, or ornamented with radiating or 



concentric ribs ; posterior side 

 angular; ligament small and 

 prominent ; hinge-teeth 2.3, 

 diverging, transversely stri- 

 ated ; centre tooth of left valve 

 divided ; pedal impressions in 

 front of the posterior adductor, 

 and one in the umbo of the 

 left valve ; anterior adductor 

 impression close to the umbo. 

 Animal with a long and 

 Fig. 221. Tngoma pectinata* pointed foot, bent sharply, heel 

 prominent, sole bordered by two crenulated ridges ; palpi small 

 and pointed ; gills ample, the outer smallest, united behind tho 

 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. From a specimen in alcohol ; the gills sliphtly curled and contracted, 

 tliey should terminate near the margin, between the arrows which indicate the inhalent 

 and cxlialcnt currents : a, a\ adductors ; h I, ligament ; t, t', dental sockets ; o, moutJi; 

 1 1, labial tentacles or palpi ; p, pallial line ; m, margin ; /, foot ; v, cloaca. 



