Contributions to Paleontology. 



113 



marginal muscular impressions have been observed in this 

 valve. Apex emarginate, with a prominent pad or tooth- 

 like process on either side. 



The following illustrations on Plate i, will serve to give a more 

 complete idea of the form and characters of this fossil : 

 Fig. 17. Natural size. 



Fig. 18. A figure enlarged about two diameters. 

 Fig. 19. Cast of a ventral valve. 

 Fig. 20. Interior of the ventral valve. 

 Fig. 21. Interior of the dorsal valve. 



These last two figures illustrate all that has been observed in the inte- 

 riors of these shells. 



It is quite probable that specimens, preserved under more favora- 

 ble circumstances, would show more clearly the character of the 

 interior. Although it may not correspond with the Author's descrip- 

 tion of the Genus Obolella, it is as clear and distinct a representa- 

 tion of the specimens in my collection as, with much care, could be 

 given. 



This species occurs in what I regard as the lowest]known fossiliferous 

 beds of the formation at Trempaleau, Black river, and other places* 

 It is associated with Lingula ampla, Conocephalites iowensis, C. 

 chippawaensis, and Theca p rimo rdialis. Some of the layers preserve 

 fucoidal remains, ripplemarks, and evidences of beach lines. 



GENUS ORTHIS (Dalman). 



ORTHIS PEPIN A ( n. s.). 



PLATE I. FIGS. 23-27. 



This species occurs as casts of the interior, and impressions of the 

 exterior surface ; and from the latter I have obtained casts from 

 which the drawings have been made. 



Shell semielliptical. Dorsal valve convex, about three- 

 fourths as long as wide, sometimes proportionally nar- 

 rower : hinge-line as long as the greatest width of the 

 shell. Ventral valve a little more convex than the dorsal, 

 longer than wide, sometimes one-fourth longer than 

 [ Trans. v.~\ 15 



