434 MANUAL OF THE MOLLUSCA. 



were usually found in old and deformed specimens ; round 

 pearls about the size of a pea, peifect in every respect, were 

 worth £3 or £4. (Dr. Knapp.) An account of the Irish pearl- 

 fishery was given by Sir E. Eedding, 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 a hundred 

 might contain a pearl, and one pearl in a hundred might bo 

 tolerably clear. (See p. 30). 



Distribution, 420 species. North America, South America, 

 Europe, Africa, Asia, Australia. 



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



Sub-genera, Monocondylwa, D'Orbigny. M. Paraguayana, 

 PI. XVIII., Fig. 2. 



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

 valve ; no lateral teeth. 



Distribution, 6 species. South America. 



Hyria, Lam. H. syrmatophora, PI. XYIIL, Fig. 3. Synonyms^ 

 Pachyodon and Prisodon, Schum. Shell Area-shaped, hinge-line 

 straight, with a dorsal wing on the posterior side ; teeth elon- 

 gated, transversely striated. Distribution, 4 species. S. America. 



Castaxja, Lamarck. 



Type. C. ambigua, PI. XYIIL, Fig. 4. 

 Synonym, Tetraplodon, Spix. 



Shell ventricose ; trigonal; umbones prominent, furrowed; 

 hinge-teeth striated; anterior 2.1, short; posterior 1.2, elon- 



J.mmaZ with mantle-lobes united behind, forming two distinct 

 siphonal orifices, the branchial cirrated. 



Distribution, 3 species. Eivers of South America, Guiana, 

 Brazil. 



, AjsroDON, Cuvier. Swan-mussel. 



Type, A. cygneus, Fig. 208, p. 398. 



Etymology^ anoduntos, edentulous. 



Shell like unio, but edentulous ; oval, smooth, rather thin, 

 compressed when young, becoming ventricose with age. 



Animal like unio : the outer gills of a female have been com- 

 puted to contain 300,000 young shells. (Lea.) See p. 14. 



Distribution, 100 species. North America, Europe, Siberia. 



Fossil, 8 species. Eocene — . Europe. 



M. D'Orbigny relates that he found great quantities of small 

 Anodons [Bysso-anodonta Paraniensis, D'Orbigny) 4 lines in 

 length, attached by a byssus, in the Eiver Parana, above 

 Corrientes. 



