THE FAMILY OF UNIONIDjE. XIX 



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

 produced anteriorly. (An. cygnea, Lam.) 



Iridina (Lam.). Mouth lips longer than wide, not united; mantle margin united as far as the 

 foot, so that the branchial and anal apertures are closed; the outer branchia united to the mantle 

 as far as its extremity; the inner branchia entirely united to the foot; foot thick, tongue-shaped, 

 somewhat produced anteriorly. (/. cselestis, Lea.) 



Spatha (Lea). The mouth lips are longer than wide, and not united. The mantle margin is 

 closed behind, so that the branchial and anal apertures are closed; on the under margin the mantle 

 is entirely open; the outer branchia is united to the mantle all the way to its extremity; the inner 

 is not united to the foot; the foot is thick, tongue-shaped, somewhat produced anteriorly. (S. rubens, 

 Lea,) Tab. 6, Fig. 2. 



Mycetopus (D'Orbigny). The mouth lips longer than wide; the mantle margin is open to the 

 anal aperture, which is closed; the outer branchia is entirely united, or grown together; the inner 

 (?); the foot is very much produced and widened into a knob. (M. solejiiformis, D'Orb.) 



In TJie Shells of New England, by TV. Stimpson, Boston (1851), he states that, 

 in the MSS. of Prof. Agassiz, the following divisions into genera are made (of the 

 New England species) on anatomical differences, viz.: — 



Unio, Retzius, emend. Gills free from the abdominal sac, their posterior extremity attached 

 to the mantle; eggs, in the female, filling the whole extent of the outer gill; mantle fringed at 

 both siphonal openings. (Unio complanatus, Lea.) 



Eurynea, Rafinesque, emend. Gills attached to the abdominal sac, leaving no communication 

 between the pedal and the upper gill cavities; eggs received in the sacs of the posterior part of the 

 outer gill; mantle fringed at the posterior half of its ventral edge, and at the branchial and anal 

 siphons. (Unio prselongus, Bar.) 



Lampsilis, Raf., emend. Gills attached; eggs in the posterior part of the outer gill; mantle 

 fringed at both siphonal openings, and having fleshy thickenings and processes at its posterior 

 ventral edge. ( Unio radiatus, Lam.) 



Metaptera, Raf, emend. Gills attached ; eggs in posterior part of gill ; a broad expansion 

 of mantle upwards; superanal opening long; both siphonal openings fringed. {Unio alatus, Say.) 



Complanaria, Swain, emend. Gills free, united to the mantle to its margin; outer gill filling 

 entirely with eggs; superanal opening long; both siphonal openings fringed; palpi united along 

 their posterior edge for two-thirds of their length. (Alasmodonta rugosa, Bar.) 



Margaritana, Schum. Gills free from abdominal sac, their posterior extremity not united to 

 the mantle ; anal opening or region of mantle not fringed ; branchial fringed. (Alasmodonta arcu- 

 ata, Bar.) 



Alasmodonta, Say, emend. Gills attached to abdominal sac, attached also to the mantle to 



their extremit} r ; branchial siphon fringed; anal large, not fringed ; eggs (?). (Alasmodonta 



marginata, Say.) 



