SYNOPSIS OF 



1. Subgenus Triquetra,' 



Having a cardinal and lateral tooth, and 

 fumislied with two siphons. 



2. Subgenus Prisodon,^ 



Having a cardinal and lateral tooth trans- 

 versely striate, and furnished with two 

 siphons. 



3. Subgenus Unio, 



Having a cardinal and lateral tooth. 



4. Subgenus Margaritana, 

 Having one tooth (cardinal). 



5. Subgenus Plagiodon, 



Having a double transA'erse cardinal tooth 



6. Subgenus Monocondyloea, 

 Having a simple callus. 



7. Subgenus Dipsas, 



Having a linear tooth under the dorsal 

 margin. 



8. Subgenus Anodonta, 

 Having no teeth. 



9. Subgenus Columba, 



Without teeth, but having an inflected 

 palleal cicatrix. 



10. Subgenus Byssanodonta,^ 



Having uo teeth, but is always attached 

 by a byssus. 



Sympbynote — Hyria corrugata. Lc 



Non-Symphynote — Castalia ambigua. 

 Lam. 



I Sympbynote — Unio alatus. Say. 



I ISTon-Symphynote — U. pictorum. Lam. 



\ Sympbynote — Alas, complanata. Barn. 

 I IN'on-Symphynote — Al. undulata. Sary. 



\ JSTon-Symphynote — Plag. isoeardi- 

 oides. Lea. 



JSTon-Sympbynote — Mono. Paraguay- 

 ana. D ' Orli. 



Sympbynote — Dipsas plicatus. Leacli. 



Sympbynote — An. magnifies. Lea. 

 JSTon Sympbynote — An.fluviatilis. Dill. 



I^on-Symphynote — Columba Blainvil- 

 liana. Lea. 



Non-Symphynote — Byssanodonta Pa- 

 vanensis. D ' Orh. 



' Since the publication of the second edition of this SynojDsis, 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 

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

 iuilections behind." 



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

 (Castalia, Lam.). Mantle ojDen 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 furnished with cilise round its edge. 

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



^ This very remarkable fresh-water bivalve, Byssanodonta Paranensis, by D'Orbigny (Voy. dans 



