OSTRACODA. 641 



Schmidtella afflnis.] 



SCHMIDTELLA AFFINIS, ». Sp. 

 PLATE Xmi, FIGS. 45-47. 



Size.— (R. V.) Length 0.97 mm.; hight 0.72 mm.; thickness 0.22 mm. 



The largest valve seen has a length of 1.08 mm. and a hight of 0.9 mm. 



This species is closely related to S. crassimarginata, and at first I was inclined 

 to view it as a later variety of that species. But, considering the great constancy 

 which prevails among the hundreds of valves of S. crassimarginata which I have 

 seen, and the equal constancy exhibited by S. affinis, it has been thought best to hold- 

 them as distinct. Besides I found it difficult to decide to which of the two, S. crassi- 

 marginata or S. umbonata, the present form bore the greater resemblance. Compared 

 with the first of these species, S. affinis is smaller, a trifle higher, the flat dorsum 

 narrower, the ends less equal, with the posterior extremity more strongly curved 

 and the basal outline more prominent in the middle. In an end view the profile is 

 less triangular and the ventral edge thinner. The most prominent point of the 

 surface also is more posterior, while the broad border, which is so conspicuous a 

 feature for S. crassimarginata, is scarcely developed. For comparison with S. um- 

 bonata and S. incompta, see following descriptions. 



Formation and locality.— Galend. shales, near Cannon Falls, Minnesota. 



Schmidtella umbonata, n. sp. 



PLATE XLV, FIGS. 36-38. 



Size.— (L. V.) Length 0.8 mm.; hight 0.59 mm.; thickness 0.23 mm. 

 (R. V.) " 0.8 " " 0.65 " " 0.23 " 



Valves ovate, slightly oblique, ends subequal, dorsum umbonate, projecting 

 considerably above the straight hinge line; free margin with a border, narrower 

 and less distinct on the left valve than on the right; greatest convexity near the 

 middle of the valves, the point occasionally marked by a very faint depression or 

 discolored spot. 



This abundant species is relatively longer than S. affinis, has more nearly equal 

 ends, narrower and better defined border, and more uniformly convex valves. From 

 the much larger S. crassimarginata it differs too obviously to require comparison. 

 S. subrotunda may be closely related but is much shorter and almost round. 



Formation and locality.— TJtv^i third of the Trenton shales, St. Paul and Cannon Palls, Minnesota. 

 A variety, or more likely a closely related species, occurs in great numbers on slabs of Birdseye limestone 

 collected at High Bridge, Kentucky. 



41- 



