490 unioninjE. 



region fringed with cirrhi, the anal plain. Foot large, thick, 

 tongue-shaped, somewhat produced anteriorly, not provided 

 with a hyssal groove. 



Shell equivalve, covered externally with an epidermis ; 

 pearly in the inside of the valves; hinge variable. 



Fresh- water. 



Sub-lam. UNIONIN^. 

 Foot moderate. 



Genus UNIO, Retzius. 



Outer gill united to the mantle as far as its extremity ; 

 inner gill not united to the foot. 



Shell equivalve, inequilateral, variable in shape, covered 

 with an olivaceous epidermis ; beaks usually eroded. Hinge 

 with primary teeth, and with elongated laterals; ligament 

 external, more or less elongated. Pallial impression simple ; 

 muscular scars conspicuous. 



Syn. Elliptio, Eurynea, Rafin. Mysca, Turt. Margaron 

 (part), Lea. Mya, Humph., not Linn. Lymnium, O/cen, 

 Luticola, Goldf. Cunicula, Legumia, Strains. 



Ex. U. pictorum, Linnteus, pi. 116, fig. 5. Shell, U. 

 pictorum, fig. 5, a, 5, b. 



M. Agassiz observes, that in this extensive genus of 

 fluviatile bivalve shells the gills are free from the abdominal 

 sac, their posterior extremity being attached to the mantle. 

 The eggs in the female fill the whole extent of the outer 

 gill. The inside of the valves is covered with a thick, 

 often iridescent nacre, and the character of the hinge-teeth 

 is very variable. The greatest number of species inhabit 



