504 GEOLOGICAL SURVEY OF IOWA. 



Spirifer submucronatus ( n. s.). 



PLATE IV. FIG. 3 a, b, c. 

 Spirifcr indct. : OWEN, Report on Wisconsin, Iowa and Minnesota, pi. iii, f. 9. 



SHELL small, somewhat semicircular, gibbous in the middle ; 

 the hinge line variable in length, often extending into mu- 

 cronate points. Ventral valve gibbous towards the beak, 

 abruptly sloping towards the front and sides ; mesial sinus 

 deep, subangular. Dorsal valve moderately convex, some- 

 times a little gibbous, and often nearly flat ; mesial fold 

 strongly elevated and flattened on the summit : area mode- 

 rately high, and in some specimens proportionally higher 

 than in others. 



SURFACE marked by eight to twelve or more strong abrupt 

 or subangular plications on each side of the mesial fold or 

 sinus ; mesial fold flattened or a little depressed in the 

 centre, and an incipient fold in the bottom of the sinus. In 

 well preserved specimens the surface is marked by sharp 

 elevated concentric laminae which arch abruptly upwards on 

 the plications, and which undulate on the mesial fold and 

 sinus corresponding to the line of depression and elevation. 



This species bears much resemblance to the S. mucronatus of the Hamilton group 

 of New-York, and is often mistaken for that species; but all the specimens which 

 have fallen under my observation are much smaller; the plications are more elevated, 

 and often flattened upon the summit ; the imbricating lamellae are stronger, more 

 closely araanged and more arched, while the area is proportionally higher. 



This species is apparently identical with that figured by Dr. D. D. OWEN, pi. iii, 

 f. 9, cited above ; since I know of no other similar species occurring in the locality 

 named by him. 



Fig. 3 a, b. Dorsal and ventral valves of a specimen having one hinge extremity more 

 produced than the other. 



Fig. 3 c. Cardinal view of the same specimen. 



Geological formation and locality. In calcareous shales of the age of the 

 Hamilton group of New- York : at Independence, New-Buffalo and Rock- 

 ingham (Iowa), and Rock-island (Illinois). 



