570 THE PALEONTOLOGY OF MINNESOTA. 
[Whitella megambona. 
The specimen figured has the following dimensions: From the postero-basal 
margin to the antero-cardinal angle, 33 mm.; from the same point to the umbones, 
31 mm.; from the postero-cardinal margin to the antero-basal margin, 26.5 mm.; 
greatest or posterior hight, 24.7 mm.; greatest convexity, 16.5 or 17 mm. 
/ Though comparable in a general way with a number of species referred to the 
genus, the relations are not very close in any case. In the matter of outline it agrees 
best with W. compressa and W. ohioensis, but the umbones are larger, the valves more 
convex, and the surface markings much better defined, especially on casts of the 
interior. The umbonal ridge is not strong enough for W. scofieldi, and the umbones 
too small for W. concentrica, while in the outline it differs in a similar manner from 
both of those species. Finally, in W. subcarinata and W. ventricosa Hall, sp., the 
anterior end is shorter. 
Formation and locality.—Middle third of the Trenton shales, Minneapolis, Minnesota. 
WHITELLA MEGAMBONA Whitfield. 
PLATE XLI, FIGS. 4 and 5. 
Cypricardites megambonus WHITFIELD, 1877. Ann. Rep. Geol. Sur. Wis., p. 73. Also 1882, Geol., Wis., 
Whitella megambona ULRICH, ia Aenea vol. vi, p. 384. 
Shell of medium size or less, oblique, subelliptical in outline, very gibbous, the con- 
vexity of the closed valves equalling very nearly the shorter of the oblique diameters, 
and about two-thirds of the distance from the beaks to the postero-basal extremity. — 
Umbones very large, tumid, with the beaks strongly and obliquely incurved, their 
points being brought into close proximity; umbonal ridge prominent, subangular 
near the beaks but becoming obtuse in receding from them; point of greatest con- 
vexity somewhat behind the middle of the shell. Hinge line very short, posteriorly 
passing rather gradually into the posterior margin; the latter is somewhat oblique 
and broadly rounded to the base where the outline turns rather sharply forward into 
the basal line, which continues with a uniform curve to the narrowly rounded— 
almost angular—anterior extremity; the latter projects about one-ninth of the longer 
diameter of the shell beyond the anterior side of the umbones. Surface marked by 
irregular concentric lines of growth, and distant obscure undulations. Anterior to 
the umbonal ridge the surface is rather strongly convex, while the posterior and 
cardinal slopes are flattened and exhibit along the center a more or less distinct 
sulcus. Escutcheon very short and narrow. Internal ligament supports, unusu- 
ally long, in casts leaving a well defined furrow on each side of the hinge line, 
extending from the upper part of the posterior margin almost to the beaks. Car- 
dinal teeth apparently as in W.scofieldi. Anterior muscular scar shallow, rather 
