88 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



in valva altera duo, crassi, basi conniventes ; in altera tres, trianguli 

 figuram formantes, intermedio minore, distincto. Dentes laterales 

 remoti, fere obsoleti. Ligamentum externum, breve, prominens, 

 turgidum. 



This genus was instituted by Bruguiere for the reception of an inter- 

 esting molluscum inhabiting the great rivers of Africa. De Blainville 

 includes it with the rest of the fluviatile Conchacea in his genus Cyclas ; 

 the propriety, however, of its separate arrangement has been fully esta- 

 blished by the anatomical description of Rang. 



The title of Galathsea, which was pre-occupied in reference to a group 

 of Crustacea, has been altered by De Roissy to that of Egeria, by Bow- 

 ditch to Megadesma, and by Sowerby to Potamophila ; we cannot, how- 

 ever, but sanction its adoption, as no one of the most obtuse perception 

 could fail to distinguish a genus of Crustacea from one of Tropiopodous 

 Mollusca ; although we must acknowledge, to the credit of these authors, 

 that it is contrary to the strict rules of nomenclature. 



The shell of Galathsea may be described as being equivalve, inequila- 

 teral, and nearly triangular ; the valves are thick and solid, of an opake 

 marble white within, often tinged round the margin with blue, and they 

 are covered externally with a thick, horny, olive-green epidermis, more 

 or less ornamented with dark radiating lines, but generally much eroded. 

 There are two thick cardinal teeth, united at the base, in one valve, and 

 three, in the form of a triangle, in the other, the middle of which is 

 smaller and isolated. The lateral teeth are remote and almost obsolete. 

 The ligament is external, short, prominent and swollen. 



Example. 



PI. LXIV. Fig. 1 and 2. 



Galathsea radiata, Lamarck, Anim. sans vert., new edit., vol. vi. p. 284. 



Enc. Meth., pi. 250. f. 1. 

 Venus subviridis, Gmelin. 

 Venus paradoxa, Born. 



