622 THE PALEONTOLOGY OF MINNESOTA. 



[Ouneamya truncatula! 



CtJNEAMYA TRUNOATULA, n. Sp. 

 PLATE XXXVI, FIG. 39. 



Shell of medium size, transversely somewhat elongate, the two ends of nearly 

 equal bight, with broad, compressed, nearly terminal, prominent and incurved beaks; 

 postero-cardinal region subalate, escutcheon less than half the length of the hinge. 

 Cardinal and basal margins diverging slightly posteriorly ; anterior end truncate, 

 almost vertical, the upper two-thirds sharply inflected, forming a rather narrow, 

 deep, and unusually long lunule, from whose lower end the outline slopes abruptly 

 backwards into the basal line; the latter is gently convex in the posterior half, 

 straight or very slightly sinuate in front of the middle, very obtusely angular in the 

 anterior third, and straight again when it ascends from the antero-basal angle to the 

 lower extremity of the lunule; posterior margin somewhat produced and strongly 

 rounded in the lower half, and very obliquely subtruncate in the upper. Posterior 

 umbonal ridge rather prominent, strongly rounded, not angular, curved and becom- 

 ing almost obsolete in the posterior third of the shell; cardinal slope concave, very 

 abrupt near the beaks; a narrow but distinct anterior umbonal ridge descends at 

 right angles to the hinge line from the beak to the antero-basal angle; between it and 

 the edge of the lunule a narrow sulcus; behind it a small well marked mesial sulcus 

 out of which the surface rises more gradually to the summit of the posterior umbonal 

 ridge. The most prominent point of the surface of the valves is situated on this 

 ridge somewhat above the middle of the hight and about two-fifths of the length 

 from the anterior extremity. Surface marked with nearly equal concentric undula- 

 tions or ridges. These are strongest in the mesial sulcus, somewhat flattened yet 

 distinct in the anterior sulcus, and nearly obsolete on the cardinal slope. Hinge and 

 muscular impressions undetermined. 



This species is closely related to C. coriformis described by Miller from the mid- 

 dle beds of the Cincinnati group of Ohio. So far as known C. truncatula never attains 

 the size of mature examples of that species, while its posterior end is higher, the 

 escutcheon much shorter, the basal outline more convex, and the anterior umbonal 

 ridge narrower and much less prominent. In C. coriformis the point of greatest 

 convexity is on the anterior ridge while it is on the posterior ridge in the Minnesota 

 species. The surface markings also are coarser, and the mesial sulcus deeper in the 

 former. 



Formation and locality. Middle Galena near WykofT and Pleasant Grove, Minnesota. 



