238 UNIONIDA. 
few, though specimens are abundant ; in North America both 
species and individuals abound. 
Like other fresh-water shells, the Naiades are often extensively 
eroded by the carbonic acid dissolved in the water they inhabit. 
This condition of the umbones is conspicuous in the great fossil 
Uniones of the Wealden, but cannot be detected in the Cardinie, 
and some other fossils formerly referred to this family. 
The outer gills of the female Unio are filled with spawn in the 
winter and early spring; the fry spins a delicate, raveled byssus, 
and flaps its triangular valves with the posterior shell-muscle, 
which is largely developed, whilst the other is yet inconspicuous. 
The shells of the female river-mussels are rather shorter and 
more ventricose than those of the males. 
Over 1200 recent species of Unionidee are known to science 
and ‘more than half of these are inhabitants of the rivers of the 
United States. A large proportion of the species was first 
described by Dr. Isaac Lea of Philadelphia, who has devoted 
over fifty years to the study of this family. His “‘ Observations 
on the genus Unio, with descriptions of new species,” etc., now 
comprises thirteen quarto volumes, illustrated by hundreds of 
beautiful plates. 
Unio, Retz. 
Etym.— Unio, a pearl (Pliny). River-mussel. 
Syn.—Uniomeris, Conr. (Hocene). 
Distr.—l000 sp., universal. Fossil. Cretaceous, Eocene—; 
Kurope. Triassic—; N. Amer. JU. littoralis, Linn. (exxiii, 86). 
Shell oval or elongated, smooth, corrugated, or spiny, becom- 
ing very solid with age; anterior teeth 1-2, or, 2-2, short, irregular ; 
posterior teeth 1-2, elongated, laminar. 
Animal with the mantle-margins only united between the si- 
phonal openings; palpi long, pointed, laterally attached. 
The subgenera are mainly founded upon peculiarities of form 
and ornamentation or sculpture; they are of no value except as 
conveniences for classifying the species; and Dr. Isaac Lea, the 
great authority upon this family, has discarded them altogether 
and used instead, in his admirable “Synopsis of Unionidee,” a 
division into sections by the form of the shell, and these into 
subsections by the sculpture. 
I annex the so-called subgenera, as adopted by H. and A. 
Adams, Chenu and others. 
BARIOSTA, Rafinesque, 1831. (Potamida, Swainson, 1840.) 
Shell arcuated, smooth. U. emarginatus, Lea (cxxiii, 75). 
NAIDEA, Swainson, 1840. Shell obovate or mytiliform, smooth. 
U. Modioliformis, Lea (exxiii, 76). 
NAIA, Swainson, 1840. Shell oblong, smooth. U. depressus, 
Lam. (exxiii, 77). 
