CALVIN ON DARK SHALE AND ITS FOSSILS. 729 



Dorsal valve convex, with a fairly defined, mesial sinus that is wide 

 in front and narrows rapidly toward the beak ; beak only a little less 

 l)romiuent than on ventral valve. Surface of both valves marked by 

 from 24 to 30 moderately strong, rounded striae that are separated by 

 wide furrows and multiply by bifurcation on the front half of shell. 

 Striae and furrows crossed by very minute microscopic lines. 



Length, 6 ; width, 7 ; thickness, 3^ millimeters. 



Known at present only from the dark shales at Independence. 



Orthis, sp. ? 



The surface is marked by coarse, angular striae, and a sharp angular 

 mesial ridge gives the valve a carinated appearance. From the Inde- 

 pendence shale. 



Spieifeka subumbona, Hall. 



Spirifera subumhona, Hall, Pal. N. Y. vol. iv, p. 234, pi. 32, figs. 22-30. 



The specimens under consideration agree very well in most characters 

 with some forms of Spirifera s'uhumhona, but, as will be seen from the 

 figures, they differ materially in size and in the width of the hinge-area. 

 IS'ot known to occur in Iowa except in the Independence shales. 



Atrypa hystrix, Hall. 



Airypa hystrix, Hall, Pal. N. Y. vol. iv, p. 236, pi. 53, A, figs. 15-17. 

 A. hystrix, H. & W., 23d Annual Eept. N. Y. State Cabinet, p. 225. 

 (See note at bottom of p. 726 of this Bulletin.) 



The specimen in hand was collected at Independence. 

 Atrypa reticularis, Linn. 



Atrypa reticularis of authors. 



The specimens collected represent the prevailing type as this species 

 occurs in the lower shale at Independence. It is more nearly related to 

 Eockford forms than to the forms found in the limestones only a few 

 feet above. 



Ehynchgnella ambigua, n. s. 



Shell large, transversely oval or elliptical ; valves moderately gibbous, 

 subequally convex ; mesial fold and sinus broad and well developed at 

 the anterior margiu in full-grown shells, becoming obsolete toward the 

 umbonal region. Length and width in about the ratio of 3 to 4. Ven- 

 tral valve regularly arched in the posterior part; the middle of the 

 anterior half of the valve occupied by a broad sinus, which becomes 

 deep and subangnlarly margined toward the front. A strong fold, ex- 

 tending about a third of the way to the beak, occupies the middle of 

 the sinus ; rudimentary folds appear on either side of the middle in the 

 sinus of large shells. Beak of ventral valve projecting slightly beyond 

 the other, closely incurved and appressed so as to show neither area nor 



