274 MANUAL OF THE MOLLUSCA. 



but cannot be detected in the Cardinifs, and some other fossils formerly re 

 ferred to this family. 



The outer gills of the female unionidee are filled with spawn in the winte 

 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, whils 

 the other is yet inconspicuous. The shells of the female river-mussels ar 

 rather shorter and more ventricose than the others. (See pp. 18, 34,) 



UNIO, Retz. River-mussel. 



Etym. Unio a pearl (Pliny). Ex. U. litoralis, PL 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. 1?2, p. 246.) 



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

 into a thin, elastic dorsal wing, as in Syria.* In the Pearl-mussel, U. % 

 garitiferus (Margaritana, Schum. Alasmodon, Say) the posterior teet 

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

 British pearls, is found in the mountain streams of Britain, Lapland, am 

 Canada ; it is used for bait in the Aberdeen Cod-fishery. The Scotch pear 

 fishery continued till the end of the last century, especially in the R. Tay 

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

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

 the size of a pea, perfect in every respect, were worth 3 or 4. (Dr 

 Knapp.) An account of the Irish pearl-fishery was given by Sir R. Reddin 

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

 river-beds with their open side turned from the torrent ; about one in 10 

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

 p. 38.) 



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



Fossil, 50 sp. Wealden . Europe, India. 



Sub-genera, Monocondylaa, D'Orb. M. Paraguay ana, PL XVIII. fig. , 

 Shell with a single large, round, obtuse cardinal tooth in each valve ; n 

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



Hyria, Lam. H. syrmatophora, PL XVIII. fig. 3. Syn. Pachyodon an 

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

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

 4 sp. S. America. 



* This is the species in which the Chinese produce artificial pearls by the intr 

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

 Gaskoin has an example containing two strings of pearls, and another in the Bri 

 Mus, has a number of little josses made of bell-metal, now completely coated wi 

 pearl, in its interior. 



