CONCHIFERA. 43& 
is fixed and mono-myary when adult, is locomotive and di-myary 
when young! * 
Like other fresh-water shells, the naids are often extensively 
eroded by the carbonic acid dissolyed in the water they inhabit 
(p. 81).+ This condition of the umbones is conspicuous in the 
great fossil Uniones of the Wealden, but cannot be detected in the 
Cardinic,and some other fossils formerly referred to this family. 
The outer gills of the female unionide are filled with spawn 
in the winter and early spring; the fry spins a delicate, ravelled 
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 yentricose than the others. 
Unto, Retz. River-mussel. 
Etymology, unio, a pearl (Pliny). 
Example, U. litoralis, Pl. XVIII., Fig. 1. 
_ Shell oval or elongated, smooth, corrugated, or spiny, becom- 
ing very solid with age; anterior teeth 1.2, or 2.2, short, irre- 
gular; posterior teeth 1.2, elongated, laminar. 
Animal with the mantle-margins only united between the 
sipkonal openings; palpi long, pointed, laterally attached. 
(Fig. 209, p. 399.) 
U. plicatus (Symphynota, Sw. Dipsas, Leach) has the valves 
produced into a thin, elastic dorsal wing, as in Hyria.{ In 
the Pearl-mussel, U. Margaritiferus (Margaritana, Schum. 
Alasmodon, Say, Baphia, Meusch.), the posterior teeth become 
obsolete with age. This species, which afforded the once famous 
British pearls, is found in the mountain streams of Britain, 
Lapland, and Canada: it is used for bait in the Aberdeen Cod- 
fishery. The Scotch pearl-fishery continued till the end of the 
last century, especially in the river Tay, where the mussels 
were collected by the peasantry before harvest time. The pearls 
%* In the synopsis at p. 406, it will be seen that each of the principal groups of bi- 
valves contains members which are fixed and irregular, and others which are byssi- 
ferous, or burrowing, or locomotive. 
+ Probably many of the organic acids, produced by the decay of vegetable matter, 
assist in the process. It has been suggested that sulphuric acid may sometimes be set 
free in river-water, by the decomposition of iron pyrites in the banks; but Prof. Boye 
of Philadelphia, states that it has not been detected in any river of the United States 
where the phenomenon of erosion is most notorious. 
¢ This is the species in which the Chinese produce artificial pearls by the introduc- 
tion of shot, &c., between the mantle of the animal and its shell (p. 88); Mr. Gask in 
Las an example containing two strings of pearls, and another in the Brit. Mus, has a 
number of little josses made of bell-metal, now completely coated with pearl, in its 
interior. 
Uv 
