PELECYPODA 



583 



in front, 1 in right valve also grooved; laterals wanting: circumpolar, as 

 far south as north latitude 40 degrees, but wanting in the central part 

 of the continent; often very common in running streams. 



6. Unio Retzius. Shell usually thick, oval or elongated, rounded in 

 front and pointed behind; hinge teeth well developed, there being 2 

 cardinals and 2 laterals in the left, and 1 of each in the right valve; 

 cloacal siphon fringed: circumpolar; 145 species; about 100 American 

 species, mostly in the southern states. 



XJ. gibbosus Barnes (Fig. 916, C). Shell rather elongate, thick and 

 heavy; dull brown in color; umbo marked by 5 or 6 large, wavy ridges; 



SITS 



Fig. 916 



Fig. 917 



Fig. 916 — A, Margaritana margartifera ; B. Anadonta undulata (Gould) ; 

 C, Unio gibbosus (Baker). Fig. 917 — Unio complanatus (Gould). 



lengih 8 cm.; height 35 mm.; width 20 mm.: valleys of Mississippi and 

 St. Lawrence; also Gulf slope; common. 



U. complanatus (Dillwyn) (Fig. 917). Shell ovate, compressed, 

 rather thin, very dark brown or green; interior usually pink; hinge with 

 a single large pyramidal and striated cardinal in the right valve, with a 

 vestige of a tooth in front of it and 2 pyramidal cardinals in the left 

 valve; laterals long and slightly curved; length 10 cm.; width 3 cm.: 

 Atlantic slope, south to Georgia; one of the commonest fresh-water 

 mussels. 



7. Anodontoides Simpson. Shell elliptical, thin, with a smooth, 

 bright outer surface; umbo with a few coarse, concentric ridges; hinge 

 teeth reduced to mere rudiments or wanting; inner 

 surface bluish-white: 2 species. 



A. fenissacianus (Lea) (Fig. 918). Length 74 

 mm. ; height 39 mm. ; width 29 mm. ; color grass green, 

 sometimes brown: valleys of the Mississippi and St. 

 Lawrence; common. 



8. Qtjadrula Rafmesque. Shell triangular, quadrate or rhomboidal, 

 thick and heavy, inflated; umbo high; hinge teeth well developed; anal 

 siphon with minute papillae or with none; male and female shell-like; 

 both the inner and the outer gills carry the eggs: 100 species, in North 

 America and Asia ; 73 American species, mostly in the Mississippi valley. 



Fig. 918 

 Anodontoides 

 ferussocianus 



(Baker). 



