26 LAND AND FRESH-WATER MOLLUSKS. 
U. tumidus, and Anodonta cygnea are found 
with it. 
Unio tumipus—(the swollen Fresh-water Mus- 
sel) (Pl. IL., fig. 2)—(Unio, Lat., a pearl).—The 
shell is about three inches long and one and 
a half wide, oval, solid with a thick glossy dark 
brown epidermis. The umbones are prominent ; 
the lunule is lanceolate and narrow; the lga- 
ment is short, thick, and prominent ; the anterior 
side is rounded and regularly sloping towards 
the front ; the posterior side wedge-shaped. The 
interior is bluish-white or salmon-colour. 
It inhabits canals and slow-running rivers with 
a muddy bottom, burying itself in a vertical posi- 
tion more than one-half of itslength. It extends 
as far north as Yorkshire. 
Unio prctorum—(the Painters’ Mussel) (Pl. IL., 
fig. 3).—The specific name originated in the use 
of these shells for holding colours by Dutch 
painters. The shell is less solid and of a more 
oblong form, and necessarily of greater propor- 
tionate width than that of U. twmidus. The 
epidermis is thin and beautifully coloured, of a 
shining greenish-yellow, banded with brown. 
The length is about two or three inches. It is 
associated with the last species. 
Unio MARGARITIFERUS—(the Pearl or Black 
Mussel) (Pl. IIL, fig. 15).—This species, also 
