76 THE CHICAGO ACADEMY OF SCIENCES. 
GROUP OF QUADRULA TRIGONA. 
15. Quadrula trigona Lea, pl. xv., fig. 5. 
Unio trigonus Lea, Trans. Amer. Phil. Soc., Vol. IV., p. 110, pl. xvi., 
fig. 40, 1831. 
Unio chunii Lea, Jour. Phil. Acad., Vol. V., p. 196, pl. xxvii., fig. 265, 
1862. 
Unio viddellii Lea, Jour. Phil. Acad., Vol. V., pp. 197-8, pl. xxvii., fig. 
267, 1862. 
Shell: Trigonal, thick and heavy, much inflated, broadly 
rounded before and behind, the posterior margin being pro- 
duced ventrally; dorsal margin curved, ventral margin more or 
less sinuous; surface more or less shining, roughened by lines 
of growth; umbones large, elevated, inflated, dark brown, 
directed anteriorly and unmarked except by growth lines; ante- 
rior umbonal slope short and flatly rounded, posterior slope 
strongly angled, with an excavation on each side of the angle 
which reaches from the apex to the ventral border, where it 
forms a decided beak-like projection; viewed anteriorly the 
shell is strongly heart shaped, resembling in this respect some 
cardia, particularly /socardia cor; ligament short, wide, stout, 
very dark brown or black; epidermis reddish or blackish horn, 
unmarked by rays; cardinal teeth double in the left and single 
in the right valve, very stout, generally not much elevated, 
triangular, diverging, very deeply grooved and striated; lateral 
teeth short, solid, elevated, lamellar, serrated, directed and 
curved ventrally; the right lateral and the lower left lateral 
have each a depression and rudiment of an additional tooth; 
connecting bridge thick, wide, flat, smooth; anterior adductor 
muscle scar forming a truncated oval, very deeply excavated, 
striated; posterior adductor muscle scar oval, well impressed, 
Striated; protractor pedis muscle scar wider than long, deeply 
impressed, striated; retractor pedis muscle scar oval, very deeply 
pitted, striated; dorsal muscle scars situated on the posterior face 
of the cardinal teeth and the under side of the connecting bridge, 
deep, large; pallial line impressed; cavity of the beaks ‘deep; 
nacre silvery white, more or less iridescent. 
Length, 47.00; height, 50.00; breadth, 40.00 mill. Mississippi River, Wis. 
ap Bd 50 rk 45.00; aie aiUE 1 Une if: ff 
“s 60:00; 9" 43.00; ‘s 380.00 ‘' Calumet River (Hybrid /). 
ae 
Animal; Anal opening smali, only slightly crenulated; 
branchial opening with stumpy brown papille; inner gill the 
