600 GEOLOGICAL SURVEY OF IOWA. 



Fig. 3 e. Cardinal view of the same. 



Fig. 3/. Interior of the dorsal valve, showing the cardinal process and hinge-line, the 

 shell below being exfoliated. 



Fig. 3 g. Enlargement of the fine concentric strise. 



Geological formation and localities. In the Burlington limestone, occur- 

 ring in great numbers at Burlington ( Iowa), Quincy ( Illinois), and other 

 places. 



Atliyris incrassatus ( n. s.). 



PLATE XII. FIG. 6. 



SHELL thick, suborbicular, a little wider than long. Ventral 

 valve with a prominent extended beak, gibbous on the umbo 

 and upper part of the shell, curving regularly towards the 

 lateral margins, with a broad sinus in front, which is only 

 conspicuous below the middle of the shell, and becomes 

 elevated into a sublinguiform process in front. 



SURFACE marked by concentric stride, and on the lower 

 part of the shell by imbricating lines of growth, which be- 

 come closely crowded and the shell much thickened at the 

 margin of the valves. 



Fig. 6. The ventral valve of this species. 



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

 lington, Iowa ; Quincy, Illinois ; Hannibal, Missouri, etc. 



Spirifer forbesi. 



PLATE XIII. FIG. 1. 



Spirifer forbesi : NORWOOD & PRATTEN, Journal Academy Nat. Sciences, Philadelphia. 



1854, N.S. Vol.iii, p. 73. 



SHELL of medium size, extremely transverse : hinge-line 

 extended into acute submucronate points. Ventral valve 

 moderately gibbous in the middle : sinus very shallow, oc- 

 cupied by from three to five plications and defined on each 

 side by a stronger plication, which is often more or less 

 distinctly dichotomised towards the base ; plications simple, 

 rounded, about twenty-two on each side of the mesial fold 

 and sinus ; cardinal area narrow. 



