„, ^^ ,. LAMELLIBBANGHIATA. 577 



Plethocardia subereota.] 



tHe antero-cardinal margin of the shell; posterior scar indistinct, larger than the 

 anterior, situated a short distance beneath the post-cardinal margin. Pallial line 

 faint, simple, submarginal. 



It is possible that this species is not distinct from the Cyrtodonta cordiformis of 

 Billings. His figures of that species looks so much like the shell above described 

 that I am nearly satisfied that they must be congeneric at least. It might be a 

 Whitella, but it is not a true Cypricardites. Compared with P. umbonata it appears 

 that in the Canadian shell the beaks are situated farther back from the anterior 

 extremity, the umbonal ridge is rounded instead of angular and the outline different, 

 especially that of the posterior end, which is also wider. 



Formation and locality. — Upper part of the middle third of the Trenton shales six miles south 

 of Cannon Falls, Minnesota. Also la cherty limestones of the age of the Black River limestone of 

 New York, in Mercer county, Kentucky. 



Plethocakdia subekbcta Ulrich. 



PLATE XL, PIGS. 25-27. 



Plethocardia suberecta Ulrich, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 245. 



Shell small, but little oblique, exceedingly ventricose, short, subelliptical in a 

 side view, with the dorso-ventral diameter much the longest. Beaks very prominent, 

 large, strongly incurved, nearly terminal; umbonal ridge strong, sharply rounded, 

 with the cardinal and posterior slopes very abrupt and nearly flat. Anterior end 

 very short, the part in front of the beaks of casts consisting chiefly of the sharply 

 defined lobe-like filling of the anterior muscular impressions. Anterior and poste- 

 rior margins gently convex, subparallel; ventral edge sharply rounded. Hinge line 

 short, scarcely extending posterior to the umbonal ridge, as seen in a side view. In 

 the casts there is a depression beneath each beak that is prolonged on each side 

 around the muscular scar. The escutcheon' seems to have been narrow, but the 

 internal ligament supports at the posterior- end of the hinge line have left two 

 strong grooves, one on each side. 



This species, though clearly congeneric with P. umbonata, is so readily distin- 

 guished that comparisons are unnecessary. 



Formation and loealiti/.— Galena, shales near Cannon Falls, Minnesota. 



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