BIVALVIA. 



97 



Unio, Betzitis, 



Mya (sp.). Linn., 1747. 



TRiauETEA. Klein, 1753.* 



LlMN^A and LlMN^ODERMA (sp.). Poli., 1791. 



Cristakia. Schum., 1817. 



Paxyodon. Id. „ 



Prisodon. Id. „ 



Margaritaxa. Id. ,, 



Amblema. Rafnesque, 1819.' 



1788. 



1820. 



Syntoxia. Rafinesque, 1820. 

 Alasmodonta. Say, 1820. 

 Mysca. Turton, 1822. 

 DiPLODON. Spix, 1827. 

 Tetraplodon. Id. „ 

 Lasmigona? Rafin., 1831. 

 Lasmonos 1 Id. „ 

 MONOCONDYL^A. D'Orb., 1835. 

 MgIiW. Swainson, 1840. 

 Cantuyria. Id. „ 



Generic Character. Shell equivalve, inequilateral, generally thick and externally 

 smooth, occasionally ornamented with nodules or spines. In the recent state covered 

 with an epidermis, often wanting at the umbones, where the shell is sometimes much 

 eroded. Hinge, with two cardinal teeth in the left valve, and one, sometimes two, in 

 the right, an elongated lateral tooth on the posterior side. Impressions by the 

 adductor muscles large and deep, that by the mantle without a sinus. Ligament 

 external. Shell nacreous. 



Animal of the form of the shell, its mantle open in front, with simple edges or 

 slightly fringed ; siphonal tubes short, plain or fringed, sometimes scarcely defined ; 

 foot large, compressed. 



This is pre-eminently a fresh-water genus, and although found living in the rivers 

 of Europe, Asia, and Africa, appears to be only fully developed in the lakes and 

 rivers of North America, more than 200 species have been named and described from 

 that part of the world. 



* In compliance with the recommendation of the Committee appointed by the British Association for 

 the Advancement of Science, 1842, "to consider the rules by which the Nomenclature of Zoology may be 

 established on a uniform and permanent basis," — the 12th edition of Liniiieus's ' Systema Naturse,' 1767, 

 is made the starting point from which the dates of priority have been adopted. 



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