or 
bo 
— 
THE PALEONTOLOGY OF MINNESOTA. 
[Aristerella. 
COLPOMYA DEMISSA, 2. Sp, 
PLATE XXXVI, FIGS. 21 and 22. 
Shell small, gibbous, arcuate, subtriangular, very high posteriorly; hinge line 
very slightly arcuate, nearly as long as the shell, forming an angle where it joins 
the nearly erect and broadly rounded posterior margin; ventral margin abruptly 
rounded and much produced in the posterior third, then ascending rapidly with a 
broad yet distinctly concave curve into the narrow anterior end, which is most 
prominent above where it turns sharply into the hinge line. Beaks of moderate 
size, compressed, incurved, about one-sixth of the length of the shell from the 
anterior extremity; umbonal ridge prominent, strongly rounded, curved. Mesial 
sulcus broad and deep, occupying the greater portion of the ventral slope. Cardinal 
slopes slightly concave, somewhat compressed and alate posteriorly. Surface with 
distinct subequal concentric striz. Hinge and interior unknown. 
The prominent umbonal ridge and deep mesial sulcus are the characters that 
have induced me to place this peculiar little shell with Colpomya. Compared with 
the other species of this genus, it will be found to differ in the much greater hight 
of its posterior end. Of Minnesota species only Modiolopsis concava is at all similar, 
but even here there is scarcely a possibility of confusion, that species being more 
elongate, its anterior end much narrower and the posterior outline quite different. 
Formation and locality.—Middle third of the Trenton shales, Chatfield, Minnesota. 
Genus ARISTERELLA, n. gen. 
Shell small, almost smooth, subovate, moderately convex, inequivalved, the left 
valve smaller than the right. No mesial sulcus. Muscular and pallial impressions 
as in Aclinomyt. “Hinge plate apparently very thin and edentulous. 
This Sans | is f founded upon a single species, which might have been placed into 
either eee or Eurr ‘ymya were it not for its unequal valves. 
ARISTERELLA NITIDULA, ”. Sp. 
PLATE XXXV, FIGS. 30—39, 
Shell small, 5 to 8 mm. long, subovate, narrowest anteriorly; hinge line nearly 
straight, long; posterior margin slightly oblique, broadly rounded, subangular at the 
extremity of the hinge; basal margin gently convex, ascending into the narrowly 
rounded anterior end. Beaks situated about one-fifth of the length of the shell from 
the anterior extremity, small, projecting slightly above the hinge, and that of the 
