566 THE BALEONTOLOGY OF MINNESOTA. ^Whitella quadrangularls. 



in front of the beaks . Anterior muscular scar elongate. Hinge thin, simple posterior 

 to the beaks, in front of them, with one long and slender horizontal tooth and 

 several slightly oblique short te'eth above it. The dimensions of a cast of the inte- 

 rior, of the average size, are as follows: greatest length, 50 mm.; greatest hight, 38 

 mm.; greatest convexity, 24 mm. A large specimen is 59 mm, long and 42 mm. high. 

 In W. sterlingensis M. and W. sp., the umbonal ridge is much stronger, the cardinal 

 area much wider, the anterior end short, the posterior margin different, especially 

 below where it is narrower, and the length from the beak to the postero-basal 

 extremity comparatively greater. W. quadrangularis Whitfield, sp., is a more convex 

 shell, iaot so oblique, and has a wider cardinal area, and larger beaks. For compari- 

 son with Trenton species see their descriptions. 



Formation and locality. —Hnason Kiver group^^.near Spring Valley, Minnesota. In Ohio and Indiana 

 the species is not uncommon in the upper beds of the Cincinnati group. 



Whitella quadrangularis Whitfield. 



PLATE XL, FIGS. 28-30. 



Cypricardites quadrangularis Whitfield, 1878. Jour. Cin. SOc. Nat. Hist., vol. i, p. 138. 



Shell of medium size, gibbous, rather erect and nearly rounded or quadrangular 

 in outline,.with very large incurved, though widely separated, subcentral beaks, over- 

 hanging the proportionally short but unusually wide ligamental areas. Length and 

 hight subequal, the latter probably a little the grea;ter; thickness more than bwo- 

 thirds of the hight. Umbonal region very prominent, rounded anterior to the 

 obtusely angular and rather inconspicuous umbonal ridge; behind the ridge the 

 surface is a little concave and slopes abruptly toward the magin; anterior slope 

 similarly abrupt and concave. Anterior end sharply rounded and most prominent 

 at the extremity of the hinge; ventral and posterior margins sometimes forming a 

 regular semicircle, but usually a slight prominence is perceptible in the postero- 

 basal regions, causing a straightening of the posterior margin. Surface marked 

 with somewhat irregular concentric lines and wrinkles of growth. 



In casts of the interior the anterior muscular scar i^ uncommonly well defined 

 for the genus, and immediately above them, a pair of depressions forming the ante- 

 rior end of the cardinal area, is also an unusual feature. Furthermore, a slight 

 vertical furrow on the umbones reminds of Cyrtodonta. Yet, despite these peculiar- 

 ities, I am convinced that the species belongs to Whitella rather than to Cyrtodonta. 

 This view is strengthened by the facts that the shell was very thin and covered on 

 the inner side by a delicate pearly nacre, parts of which are preserved on the cast 

 represented by figure 29. Such a film has been observed on casts of other species of 

 Whitella, but has never been noticed on similarly preserv.ed species of Cyrtodonta. 



