524 THE PALEONTOLOGY OF MINNESOTA. 



[Aristerella. 



COLPOMYA DEMISSA, n. 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 striae. 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 bight 

 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, Chatfleld, 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 \r\..Aetmomyar Hinge plate apparently very thin and edentulous. 



This genus is founded upon a single species, which might have been placed into 

 either Actrnemya. or Eurymya were it not for its unequal valves. 



AEISTERELLA NITIDULA, n. 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 



