TRIGONIIDA. 945 
BartvettTia, H. Adams, 1866. 
Distr.—B. Stefanensis, Moric cxxiv, 7). Amazon River. 
Shell free, equivalve, closed, inequilateral, the anterior por- 
tion being peculiarly produced and rugose, the ventral edge 
insinuated, hinge edentulous; ligament marginal, partially 
internal, supported by strong fulcra, muscular scars two, mar- 
ginal; pallial line entire. 
This singular shell has the irregular growth of the attached 
genera, except that it is equivalve; in possessing two muscular 
sears it is nearest allied to Atheria. 
( Trigoniacea. ) 
Famity TRIGONIID®. 
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. 
TRIGoNIA, Bruguiére. 
Etym.— Trigonos, three-angled. 
Syn.—Lyriodon, G. Sowerby. Myophorella, Bayle. 
Distr.—3 sp. (or varieties?). Australia. Fossil, 100 sp. 
Devonian—; Europe, United States, Chili, Algeria, Cape, 
South India. 7. pectinata, Lam. (cxxv, 18). 
Shell thick, tuberculated, or ornamented with radiating or 
concentric ribs; posterior side angular; ligament small and 
prominent; hinge-teeth 2°3, diverging, transversely striated ; 
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 pointed foot, bent sharply, heel promi- 
nent, 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 inte- 
rior are called “ horse-heads ” by the Portland (England) quarry- 
men; they spoil the stone. Silicified casts have been found at 
Tisbury, in which the animal itself, with its gills, was preserved. 
The species with the posterior angle of the shell elongated have 
a siphonal ridge inside. The epidermal layer of the recent shell 
consists of nucleated cells, forming a beautiful microscopic 
object. A Trigonia placed by Mr. 8. Stutchbury on the gunwale 
of his boat leapt overboard, clearing a ledge of four inches; they 
