70 THE CHICAGO ACADEMY OF SCIENCES. 
GROUP OF UNIO GIBBOSUS. 
11. Unio gibbosus Barnes, pl. xiv., figs. 3, 4; pl. xv., figs. 1, 2, 3. 4. 
Unio gibbosus BARNES, Am. Journ. Sci. and Arts, Ist series, Vol. VI., 
jolee dt fig. 12, 1823. 
Unio arctior Lea, Trans. Am. Phil. Soc., Vol. VI., p. 10, pl. iv., fig. 10, 
1834. 
Unio dilatatus RAFINESQUE, Conrad in monograph, pl. xxi., 1838. 
Shell: Of medium size, elongately elliptical, thick and heavy, 
compressed, rounded before and obtusely pointed behind; 
dorsal and ventral margins slightly curved or rounded; surface 
decidedly roughened by growth lines; umbones not elevated, 
compressed, yellowish brown, marked by five or six large wavy 
ridges; the umbones are directed anteriorly; anterior umbonal 
slope rounded; posterior slope rather flat, forming an angle with 
the sides of the shell; ligament long, rather wide, stout, dark 
horn or black; epidermis brownish or blackish, plain, sometimes 
reddish, and showing very light traces of rays; cardinal teeth 
small, double in both valves; the anterior tooth in the right 
valve very small, the posterior tooth elevated, triangular, thick, 
serrated, in the left valve equal, elevated, pyramidal, striated; 
lateral teeth long, lamellar, thick, serrated, sometimes tending 
to become double in the right valve; anterior adductor muscle 
scar large, deeply impressed, very strongly striated; posterior 
adductor muscle scar longer than wide, rather lightly 
impressed, concentrically striated; protractor pedis muscle scar 
wider than long, deeply impressed, striated; dorsal muscle scars 
situated below the hinge line posterior to the cardinal teeth, 
deeply impressed; pallial line impressed on the anterior part of 
the shell, crenulate; cavity of the beaks very shallow; nacre 
silvery white, salmon or purple, the purple being frequently 
confined to the center of the shell, below the hinge. 
Length, 82.00; height, 36.00; breadth, 20.00 mill. ( (8423). 
“ 66.00: “ 84.00; ‘ 20.00 “ Q (7388). 
“73.00; “ 33.00; ‘ 19.00 “ (10087). 
“85.00; =‘ 85.50; “ 19.00  @ (12412). 
Animal: Yellowish brown in color, shading into creamy 
white; near the anterior part, the mantle edged with black, 
especially about the siphonal portion; anal opening of medium 
size with small papilla; branchial opening larger with good 
sized papille; ctenidia long, rather wide, ends rounded, 
united above throughout their entire length, the outer ctenidium 
the smaller; labial palpi small, triangular, united at base, and 
