BIVALVES. 41 x 
are unknown. North America, and especially the drainage-area of the Mississippi 
and its tributaries, is the great home of the Unionidce. The species may 
be counted by hundreds, some being the most remarkable and beautiful that 
exist in any part of the globe. The shells are usually equivalve, and joined by 
an external ligament, but exhibit great variation in shape. The hinge is some¬ 
times destitute of teeth (. Anodonta , Mycetopus ); or powerfully formed with 
strong complex interlocking teeth, as in many of the North American forms; or 
it may consist of very numerous teeth on a straight hinge-line ( Pliodon ), 
recalling the form of hinge obtaining in the Arcidce. The exterior is covered with 
a thick, often glossy periostracum, varying in colour, the prevailing tints being 
large river-mussel, Anodonta cygnea (11 at. size). 
greenish olive, brownish yellow, brown, and black. Many are beautifully rayed 
with green. The solidity of many of the species—especially the North American 
forms—is remarkable, although others are thin and fragile. Some are pearly 
within, and others white, pinkish purple, salmon-colour, yellow, or iridescent. 
The shells are marked with two adductor scars, and the pedal scars are also often 
clearly visible, while the pallial line is uninterrupted by a posterior sinus. The 
animal of Unio has the lobes of the mantle free, excepting posteriorly, where 
they are connected, forming two orifices, the lower or branchial for the passage of 
the water to the gills, and the upper for excretal purposes. The former is 
fi’inged with several rows of papillae. The foot is large, thick, tongue-shaped, 
and used as a creeping and burrowing organ. The sexes are united in the 
European species, but distinct in the American. A remarkable feature in con- 
