xviii SYXOPSIS or 



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

 according to the character of the shell. 1 



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

 pied, et d'apres les caracteres seuls de la coquille. On pourrait les diviser ainsi qu'il suit.'' 



1. Manteau ferine" jusqu'au tiers inferieur de la coquille, pourvue des deux tubes distincts, &c. 

 = Tridina, Lam. 



2. Manteau ferae" seulment sur la region anale, ou il y a deux tubes distincts, &c. = Castalia, 

 Lam. 



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

 D'Orb. 



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



A. Coquille pourvue des dents variables a la charniere = Unio, Retz. 



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



C. Coquille sans dents a la charniere = Anodonta, Larn. 



Prof. Troschel, in Wiegmanri's Archives, vol. xiii. 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 open; 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, Fig. 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 ambiyua, Lam.) 



DIPSAS (Leach). Animal unknown. Shell closed entirely (Dipsas plicatus, 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 



1 M. D'Orbigny, in his Voy. dans 1'Am. Mer., v. 5, pt. 3, p. 502, says that the eleven genera made by 

 Agassi/ out of the Myse may all be placed in existing genera. 



