OSTRACODA. 641 
Schmidtella affinis.] 
SCHMIDTELLA AFFINIS, 7. Sp. 
PLATE XLIII, FIGS. 45—47. 
Sizze.—(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. wm- 
bonata and S. incompta, see following descriptions. 
Formation and locality.—Galena shales, near Cannon Falls, Minnesota. 
SCHMIDTELLA UMBONATA, %. Sp. 
PLATE XLV, FIGS. 36—38. 
Size.—(L. V.) Length 0.8 mm.; hight 0.59 mm.; thickness 0.23 mm. 
(R. V.) SS) SJ0'8= SOF GOIN ss OLB re 
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..—Upper third of the Trenton shales, St. Paul and Cannon Falls, 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— 
