640 THE PALEONTOLOGY OP MINNESOTA. „ ... 



[Sohmldtella crassimargiaata. 



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 ? ohsoletus and A. ohlongus 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 XLllI, FIGS. 42-44. 

 Schmidtella crassimarginata, Ulkich, 1892, Amer. GeoL, vol. x, p. 269. 

 Size.— (E. V.) Length 1.80 mm.; hight 1.45 mm.; thickness 0.60t 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 j-ust 

 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.— hovfei Trenton limestone, Mineral Point, Wisconsin, and Dixon, Illinois. 

 Its occurrence in this limestone at Minneapolis is not yfet established with certainty. 



*Zeitschr. d. Deutsch. geolog. Gesellschaft,1891, 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. 



