374 MANUAL OF THE MOLLUSCA. 



but cannot be detected in the Cardinice, and some other fossils foimerly re- 

 ferred to this family. 



The outer gills of the female unionidse are filled with spawn in the winter 

 and early spring ; the fry spins a delicate, ravelled byssus, and flaps its trian- 

 gular 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 the others. (See pp. 18, 34.) 



Unio, Retz. River-mussel. 



Etym. Unio a pearl (Pliny). Ex. U. litoralis, PI. XVIII. fig. 1. 



Shell oval or elongated, smooth, corrugated, or spiny, becoming 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 siphonal open- 

 ings ; palpi long, pointed, laterally attached. (Fig. 172, p. 246.) 



U. plicatus (Symphynota, Sw. Dipsas, Leach) has the valves produced 

 into a thin, elastic dorsal wing, as in Hyna* In the Pearl-mussel, TJ. mar- 

 garit'iferus (Margaritaua, Schum. Alasmodon, Say) the posterior teeth 

 become obsolete with age. This species, which afforded the once famous 

 British pearls, is foimd 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 centmy, especially in the R. Tay, 

 where the mussels were collected by the peasantry before harvest- time. The 

 pearls were usually found in old and deformed specimens ; round pearls about 

 the size of a pea, perfect in every respect, w^ere worth £3 or £4. (Dr. 

 Knapp.) An account of the Irish pearl-fishery w^as given by Sir R. Redding 

 in the Phil. Trans. 1693. The mussels were found set up in the sand of the 

 river-beds with their open side turned from the torrent ; about one in 100 

 might contain a pearl, and one peaii in 100 might be tolerably clear. (See 

 P-38.) 



Distr. 250 sp. N.America, S. America, Europe, Africa, Asia, Australia. 



Fossil, 50 sp. Wealden — . Europe, India. 



Sub-genera, Monocondylcea, D'Orb. M. Paraguayana, PI. XVIII. fig. 3. 

 Shell with a single large, round, obtuse cardinal tooth in each valve ; no 

 lateral teeth. Distr. 6'sp. S. America. 



Hyria, Lam. H. syrmatophora, PI. XVIII. fig. 3. Syn. Paohyodon and 

 Prisodon, Schum. Shell Area-shaped, hinge-line straight, with a dorsal 

 wing on the posterior side; teeth elongated, transversely striated. Distr. 

 4 sp. S. America. 



» This is the species in which the Chinese produce artificial pearls by the intro- 

 duction of shot, &c., between the mantle of the animal and its shell (p. 38); Mr. 

 Gaskoin has 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. 



