LAMELLIBEANCHIATA. 543 



Cyrtodonta glabella. 



the part that is in front of the beaks. Severalspecies 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 Gannon Falls, 

 Minnesota. 



Cyrtodonta glabella TJlrich. 



PLATE XXXIX, PIGS. 37 and 40. 



Oypricardites glabella Ulrich, March, 1892, Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn.,. 



p. 234. 

 Oypricardites minnesotensis Sakdeson, 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 im 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 sulcus 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 tTie 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. 



