TRIGONIA. DIMY ARIA. 91 
seldom distinctly marked : ligament linear, external, sunk 
in a cleft at the anterior extremity. 
Anadon Cordierti, Plate XI. fig. 2. Found in the Cal- 
caire-grossier at Paris, 
The shells of this genus are fluviatile, inhabiting lakes, rivers, and canals in 
all countries, but they are chiefly found in still waters, adhering to the mud 
at the bottom, generally a little sunk into it. They are at once distinguished 
from their congenerous genera, Hyria, Unio, Iridina, and Megadesma, by 
being totally devoid of teeth. 
Fossil Anodons are very rare. 
Genus IV. — UNIO. — Bruguiere. 
Generic Character. — Shell transverse, equivalve, inequi- 
lateral, free, sometimes subcordate, or suborbicular ; pearl- 
aceous within ; generally covered with a dark olivaceous 
epidermis, which is usually decorticated on the umbones, 
which are prominent ; hinge provided with a short, irregular, 
simple, or a double compound tooth, which is almost always 
striated ; with two elongated, compressed, lateral teeth, the 
front one produced, sometimes obsolete ; two muscular im- 
pressions in each valve, the superior. one compound, or 
composed of several divisions ; ligament external, 
Unio subtruncatus. Plate XL. fig. 3. 
The teeth vary considerably in their progress from the young to the adult 
condition. 
Fossil species are numerous. 
The species of this genus are very numerous, inhabiting lakes, rivers, and 
canals in almost every quarter of the globe. The great rivers of America are 
very rich in species. They vary considerably in their external form, being 
sometimes nearly cordate ; for the most part thick, and oftentimes very pon- 
derous for their size. 
. TRIBE II.— TRIGONACEA. 
Primary teeth lamelliform, and transversely striated. 
Genus V.— TRIGONIA. — Bruguiére. 
Generic Character. — Shell equivalve, inequilateral, trans- 
verse, trigonal, sometimes suborbicular; cardinal teeth 
oblong, laterally compressed, divergent, two in the right 
valve transversely grooved on both sides; the grooves 
regularly marked, each forming the segment of a circle ; 
four teeth in the left valve grooved in one side only, but 
these alternately in pairs; consequently the four teeth of 
this valve receive within their grooved sides the two teeth 
of the right valve ; two principal muscular impressions, the 
