570 THE PALEONTOLOGY OF MINNESOTA. 



[Wliitelhi. niegainbona. 



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. scqfieldi, 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., 



vol. iv, p. 210. 

 WJiitella megambona ULRICH, 1890. Amer. Geol., 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 



