eo 
LAMELLIBRANCHIATA. 54 
Cyrtodonta glabella. 
the part that is in front of the beaks. Several species of Vanuxemia present a similar 
external appearance, but they have all a thicker shell and are quite different intern- 
ally, so that casts of the interior could not possibly be confounded. 
Length, 14.2 mm.; from umbone to postero-basal margin, 14.8 mm.; hight at 
middle of shell, 13 mm.; thickness, 11 mm. 
Formation and locality.—Base of the middle Galena, about thirteen miles south of Cannon Falls, 
Minnesota. 
CyRTODONTA GLABELLA Ulrich. 
PLATE XXXIX, FIGS. 37 and 40. 
Cypricardites glabella ULRICH, March, 1892, Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., 
Cypricardites minnesotensis enna April. 1892. Bull. Minn. Acad. Nat. Sci., vol. iii, p. 338. 
Shell of medium size, moderately convex; broad ovate or subquadrangular in 
outline, with the back straight and rather long, the posterior margin broadly rounded, 
sometimes nearly vertical and slightly straightened in the middle, above making 
an obtusely angular or more or less rounded junction with the hinge line; ventral 
and anterior margins rounded, the latter turning rather sharply backward at the 
hinge. Beaks situated well forward, small, very slightly prominent, the umbonal 
region full, with the line of greatest convexity—not sufficiently defined to be called 
a ridge—extending obliquely across the valve from the beaks. Cardinal slope flat, 
rather abrupt; between this and the undefined umbonal ridge, the surface is again 
flattened; anterior and basal slopes gently convex. Surface marked with somewhat 
irregular concentric lines of growth. 
Good moulds of the interior show that the hinge plate was strong, the liga- 
mental area very narrow, the cardinal teeth at least two and strong, and the poster- 
ior teeth two or three iw each valve. The beaks are prominent, incurved, and com- 
pressed because of a sulcus that crosses the valves a little obliquely, but is lost before 
reaching one-half the distance to the ventral border. On each side of the suleus is a 
very faint ridge. Anterior adductor distinct, rather small, ovate, acuminate below. 
Pallialline distinct, especially the anterior part where it appears as a sharply 
defined pustulose ridge in the cast. Posterior adductor ovate, the long diameter 
vertical, nearly three times the size of the anterior, situated about one-third of its 
length beneath the posterior end of the hinge plate. 
This fine shell is an early form of the group of species of which C. germana, 
C. grandis and C., billingsi are more typical representatives. It is distinguished 
from them all by the more anterior position of the beaks, and greater prominence 
of the antero-basal margin. The next species, though very similar in most respects, 
belongs to another group of species, in which the internal ridge and sulcus is 
indistinguishable. 
