COXCHIFERA. 433 



is fixed and mono-myary wlieii adult, is locomotive and di-myary 

 when young ! * 



Like other fresh-water shells, the naids are often extensively 

 eroded by the carbonic acid dissolved in the water they inhabit 

 (p. 31). t This condition of the umbones is conspicuous in the 

 great fossil Uniones of the Wealden, but cannot be detected in the 

 Cardinice, and some other fossils formerly referred to this family. 



The outer gills of the female unionidte 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 ventricose than the others. 



Unio, Eetz. Eiver-mussel. 



Etymology, unio, a pearl (Pliny). 



Example, U. litoralis, PI. XYIII., 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 

 siphonal openings ; palpi long, pointed, laterally attached. 

 (Fig. 209, p. 399.) 



U. plicahis (SjTiiphynota, Sw. Dipsas, Leach) has the valves 

 produced into a thin, elastic dorsal wing, as in Hyria.X In 

 the Pearl-mussel, U. Margaritiferus (Margaritana, Schum. 

 Alasmodon, Say, Baphia, Meusch.), the posterior teeth become 

 obsolete with age. This species, which afi'orded 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. 



t 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 pjTites in the banks ; but Prof. Bnye 

 of Philadelphia, states that it has not been detected in any river of the United letates 

 where tTie phenomenon of erosion is most notorious. 



t 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. 38) ; Mr. Gask in 

 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. 



U 



