238 Twenty-third Report ox the State Cabixet. 



Spirifera cyrtixaformis n. sj). 



Plate 11, figs. 21-34. 



Shell small, subtrapezoidal, with a slightly convex dorsal valve, 

 marked by a faint, undefined, mesial elevation and a highly pyramidal 

 ventral valve. Cardinal area vertical, nearly as high as wide, divided 

 nearly or quite to the apex, by a somewhat large foramen, which is 

 distinctly grooved on the edges for the reception of a detidial plate. 

 Surface of the valves marked by about forty-live to fifty low, rounded 

 plications ; the central plication on the mesial fold and sinus divides 

 once or twice before reaching the front of the shell ; the other pli- 

 cations are simple. 



This shell has all the external appearances of Cyrtina, except in the 

 want of a closed deltidium. Internally, it is without the mesial septum 

 of the ventral valve of the shells of that genus ; and the spires are 

 arranged as in Spirifera. Under the microscope the shell proves to 

 be impunctate. 



Formation and locality. In the marly beds at Rockford and Hack- 

 berry, Iowa. 



Genus — PLEUROTOM ARIA De France. 



Pleltjotomaria Isaacsii n. sj). 



Plate 13, figs. 6, 7. 



Shell depressed, suborbicular, with moderately elevated spire and 

 rapidly increasing volutions, three to four in number, flattened or 

 slightl}" convex on the upper surface, rounded below, ol)liquely truncate 

 on the periphery. Umbilicus very broad and open. Surface of shell 

 apparently destitute of ornamentation. 



This species bears much resemblance to the P. arata Hall, from the 

 Schoharie grit of New Fork, and may be considered as a western rep- 

 resentative species appearing to hold a very similar geological position. 

 It difiers in the absence of the transverse furrows, crossing the upper 

 side of the volutions, which characterize P. arata. 



Formation and locality. From the limestone beds which appear to 

 lie beneath tlie Waterloo beds ; near Raymond station. Collected and 

 presented by Mr. Mclsaacs, of Waterloo, Iowa. 



Genus — NATICOPSIS McCoy. 

 Naticopsis gigantea n. sp. 



Plate 12, flgs. 8-10. 



Shell large and thick, ovate-conical, with a moderately ascending 

 spire which forms a little more than one-third of the entire length. 

 Volutions somewhat angular, flattened on the sides and below, with 

 strong, deep sutures. Aperture oblique, somewhat ])atulose, slightly 

 extended below ; columella lip thickened and flattened on the lower 



