XXVI SYNOPSIS OP 



1. Subgenus Triquetra, 1 f 



Having a cardinal and lateral tooth, and j Symphynote Hyria COlTUgata. Lam. 

 furnished with two siphons. 



a 



Non-Symphynote Castalia ambigua. 



2. Subgenus Prisodon, 2 



Having a cardinal and lateral tooth trans- 

 versely striate, and furnished with two 

 siphons. 



3. Subgenus Unio, [ Symphynote Unio alatns. Say. 

 Having a cardinal and lateral tooth. { Non-Symphynote U. pictorum. Lam. 



4. Subgenus Margaritana, J Symphynote Alas, complanata. -Barn. 

 Having one tooth (cardinal). [ Non-Symphynote Al. undulata. Say. 



5. Subgenus Plagiodon, f ]S"on-Symphynote Plag. isocardi- 

 Having a double transverse cardinal tooth Oldes. Lea. 



6. Subgenus Monocondyloea, [ Non-Symphynote Mono. Paraguay- 

 Having a simple callus. aa. D ' Orb. 



7. Subgenus Dipsas, f 



Having a linear tooth under the dorsal Symphynote Dipsas plicatus. Leach. 

 margin. 



8. Subgenus Anodonta, f Symphynote An. magnifica. Lea. 

 Having no teeth. | Non Symphynote An.fluviatilis. Dill. 



9. Subgenus Columba, f __ 



JNon-Symphynote Columba Blamvil- 



Without teeth, but having un inflected i ljo n T ea 

 palleal cicatrix. 



10. Subgenus Byssanodonta, 3 ,, 



Json-Symphynote Byssanodonta 1 a- 



Ilaving no teeth, but is always attached 

 by a byssus. 



rauensis. D'Orb. 



1 Since the publication of the second edition of this Synopsis, the soft parts of Triquetra (Hyria, 

 Lam.) have been obtained, and Mr. Gray, in the An. and Mag. Nat. Hist., vol. vi., gives us the anatomy. 

 " The mantle lobes of the species of this genus brought from British Guiana by Mr. Schomburgh are 

 united together behind, and furnished with two short, separate, contractile siphons, like the animals of 

 Iridina and Leila, though the submarginal impression of the shell does not show indications of any 

 inflections behind." 



9 M. D'Orbigny, in his Voy. Am. Mer., vol. v. p. 597, gives a description of the soft parts of Prisodon 

 (Castalia, Lam.). Mantle open the whole length, except at the anal region, where it is closed, and pre- 

 sents two short distinct tubes, of which one branchial is larger and famished with cilia: round its edge. 

 Buccal appendages rounded, very large. Foot much compressed, thicker and bent behind. 



1 This very remarkable fresh-water bivalve, Byssanodonta Paran<'n*ix, by D'Orbigny (Voy. dans 



