Xviii SYNOPSIS OF 



the family into genera according to the form of the mantle, the form of the foot, and 

 according to the character of the shell.^ 



"Qui se divisent zoologiquement en plusieurs genres d'apres la forme du manleau, la forme du 

 pied, et d'apres les oaract^res seals de la coquille. On pourrait les diviser ainsi qu'il suit." 



1. Manteau ferm^ jusqu'au tiers inf^rieur de la coquille, pourvue des deux tubes distiucts, &c. 

 = Iridina, Lam. 



2. Manteau fermd seulment sur la region anale, oii il y a deux tubes distincts, &c. = CAstalia^ 

 Lam. 



3. Manteau ouvert sur toute sa longeur, une ouverture anale seule distincte, &c. = Mycetopus, 

 D'Orb. 



4. Manteau ouvert sur toute sa longeur une ouverture anale seule distincte. 



A. Coquille pourvue des dents variables a la charni^re = Unio^ Retz. 



B. Coquille pourvue d'une seule dent = Monocondylcea, D'Orb. 



C. Coquille sans dents a la charni^re = Anodonta^ Lam. - 



Prof. Troschel, in Wiegmann's Archives, vol. xili. 1847, divides the family, by 

 their anatomical structure, into nine genera, viz. : — 



Unio (Retzius). The mouth lips are wider than long, united as far as the middle on the pos- 

 terior margin. The margin of the mantle entirely open ; the outer branchia united to the mantle 

 to its very extremity; the inner branchia not united to the foot; the foot is tongue-shaped, thick, 

 somewhat produced anteriorly. ( Unio tumidus, Retz., and Unio Hopetonensis, Lea.) 



Margaritana (Schumacher). {Alasmodonta, Say.) The mouth lips are wider than long, united 

 along two-thirds of the posterior margin. The mantle entirely opea ; the outer branchia is free 

 posteriorly, and lies in a fold of the mantle; the interior branchia is united to the foot anteriorly, 

 but along the greater part of the foot it is free; the foot is tongue-shaped, somewhat produced 

 anteriorly. {Mya margaritifera, Lin.,) Tab. 6, B'ig. 1. 



Hyria (Lam.) The mouth lips as wide as long, not united. The mantle margin inferiorly is 

 open, posteriorly closed, so as to make a branchial and anal aperture. The outer branchia united 

 to the mantle to its extremity; the interior branchia is united to the foot; the foot tongue-shaped, 

 thick, somewhat produced anteriorly. {H. syrmatophora, Lam.,) Tab. 6, Fig. 3. 



Castalia (Lam.). The mouth lips a little wider than long. The margin of the mantle infe- 

 riorly is open, posteriorly so united that the branchial and anal apertures are closed; the branchial 

 aperture is surrounded with cirri ; the anal aperture smooth ; the outer branchia united to the 



mantle all the way to its extremity; the inner branchia (?); the foot tongue-shaped, thick, 



somewhat produced anteriorly. D'Orbigny, Voy., &c. {Castalia amhigua, Lam.) 



DiPSAS (Leach). Animal unknown. Shell closed entirely {Dijisas pUcatus, Leach). 



Anodonta (Lam.). The mouth lips are broader than long, united to the middle of the poste- 

 rior margin ; the mantle margin is entirely open ; the outer branchia united to the mantle to its 



' M. D'Orbigny, in his Voy. dans I'Am. Mer., v. 5, pt. 3, p. 502, says that the eleven genera made by 

 Agassiz out of the 3Iyae may all be placed in existing genera. 



