566 THE PALEONTOLOGY OF MINNESOTA. 
[Whitella quadrangularis. 
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 teeth 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, 24mm. A large specimen is59 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, not so oblique, and has a wider cardinal area, and larger beaks. For compari- 
son with Trenton species see their descriptions. 
Formation and locality.—Hudson River 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 greater; thickness more than two- 
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, 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 preserved species of Cyrtodonta. 
