134 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



by a few radiating or vascular impressions. On each side of this 

 central elevated plate, and at the anterior extremities of the 

 cardinal line, are two smaller muscular impressions. There is a 

 narrow groove beneath the apex of the valve, and on each side 

 of this an elongate thickened boss or padi 



The dorsal(?) valve has a narrow central muscular impression, 

 the limits of which extend in an acute point below the middle of 

 the shell : on each side is a broader limited area, reaching two- 

 thirds the length of the shell, and extending into the cavity be- 

 neath the beak. The whole presents an elongate somewhat cordi- 

 form area. No marginal muscular impressions have been observed 

 in this valve. Apex emarginate, with a prominent pad or toothlike 

 process on either side. 



The following illustrations, on Plate vi, will serve to give a more com- 

 plete 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 interiors of these 

 shells. 



It is quite probable that specimens, preserved under more favorable 

 circumstances, would show more clearly the character of the interior. Al- 

 though it may not correspond with the Author's description of the Genus 

 Obolella, it is as clear and distinct a representation of the specimens in 

 my collecti(jn 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 ioivensis, G. chi2opawaensis, 

 and Theca primwi'dialis. Some of the layers preserve liicoidal remains, 

 ripplemarks, and evidences of beach lines. 



GENUS ORTIIIS ( Dalman). 



ORTHIS PEPINA (n.s.). 



PLATE VI. FIGS. 23 - 27. 



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

 rior surface ; .and from the latter, I have obtained casts from which the 

 drawings have been made. 



