640 THE PALEON'TOLOGY OF MINNESOTA. 
(Schmidtella crassimarginata. 
The species which I propose to arrange under this genus might have been placed 
with Aparchites, Jones, were it not that they have overlapping valves. Even without 
that difference it may be questioned if such an arrangement would have been strictly 
proper, since no true Aparchites is strongly developed or gibbous in the dorsal region. 
As a rule Aparchites is thickest beneath the middle of the valves. The same is 
true of Leperditia, a genus that will, I think, be admitted by all to be distinct from 
Schmidtella. Though still somewhat in doubt respecting the systematic position of 
the new genus, it seems well to place it provisionally between Leperditia and 
Aparchites. 
Besides the six Trenton species about to be described, Schmidtella will include 
Aparchites ? obsoletus and A. oblongus of the Upper Silurian rocks of Europe. I refer 
to the specimens so designated and identified by Dr. Krause* with two British species 
described by Jones and Holl under Primitia in 1865, and more recently (1889) referred 
to Aparchites by Prof. Jones. While I am inclined to question the identity of the 
British and German specimens, I can scarcely doubt that the latter at least are truly 
referable to Schmidtella. 
SCHMIDTELLA CRASSIMARGINATA Ulrich. 
PLATE XLIII, FIGS. 42—44. 
Schmidtella crassimarginata, ULRICH, 1892, Amer. Geol., vol. x, p. 269. 
SizeE.—(R. V.) Length 1.80 mm.; hight 1.45 mm.; thickness 0.607 mm. 
Valves broadly suboval, very slightly oblique, the dorsal outline more gently 
arcuate than elsewhere, ends nearly. equal though the posterior margin is more 
curved, especially above, than the anterior, the latter often forming an obtuse angle 
where it joins the dorsal line; ventral outline uniformly curved, semielliptical; back 
flattened, slightly convex in a side view, raising very abruptly from and projecting 
slightly above the nearly straight hinge-line; point of greatest thickness just 
behind the center of the upper half; a rather conspicuous yet not sharply defined 
broad furrow around the ends and ventral margin, least distinct posteriorly, produces 
the thick border that has suggested the specific name. Specimens vary in length 
from 1.6 mm. to 2.0 mm. 
The border is more distinct and wider, and the back more flattened than in any 
of the other species referred to the genus. 
Formation and locality.—Lower Trenton limestone, Mineral Point, Wisconsin, and Dixon, Illinois. 
Its occurrence in this limestone at Minneapolis is not yet established with certainty. 
*Zeitschr. d. Deutsch. geolog. Gesellschaft,1991, p. 492. 
+The dimensions given in the original description are too small, the magnification of the valve measured having been 
supposed to be 15 diameters when it was only about 10 diameters, 
