602 GEOLOGICAL SURVEY OF IOWA. 



while the outline is more rounded below and wider towards the extremities, which 

 are not so much produced as in that species. 



Fig. 2. Dorsal valve of a specimen of this species. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Spirifer incertus ( n. s.). 



PLATE XIII. Fio. 3. 



SHELL semielliptical, more than twice as wide as long : 

 cardinal extremities salient ; valves nearly equally convex. 

 Ventral valve gibbous in the middle and compressed at the 

 extremities. Dorsal valve somewhat less convex than the 

 opposite : area moderately wide, linear and extending to the 

 cardinal extremities, common to the two valves, the dorsal 

 being about half as wide as the ventral area ; foramen large. 



SURFACE marked by fine, abruptly elevated, round or sub- 

 angular plications, which cover equally the mesial sinus and 

 elevation ; those of the latter originating at the apex in a 

 single one, which frequently dichotomizes before reaching 

 the base. Plications all dichotomizing once or twice below 

 the beak, and reaching the number of thirty or more at the 

 margin of the shell ; concentrically marked by fine sharp 

 elevated imbricating lamellae. 



Both this species and the preceding ( Plate xiii, fig. 2 ) bear considerable resem- 

 blance to Spirifer attenuatus of SOWERBT; but the proportionate length and breadth 

 differ in some degree, while there are other differences in the striae, the area, etc. 

 In the absence of authentic specimens for comparison, I must presume them to be 

 distinct. 



Fig. 3. Cardinal view of the specimen, showing the linear area. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



