646 GEOLOGICAL SURVEY OF IOWA. 



Spirifer subcnspidatus ( n. s.). 



PLATE XX. FIG. 5 a, b. 



SHELL triangularly subpyramidal ; width about one-third 

 greater than the length, extended into subacute angles. 

 Dorsal valve semielliptical, moderately convex on each side 

 of the mesial fold, abruptly incurved towards the margin : 

 mesial fold narrow and well defined near the beak, rapidly 

 increasing in width and rising towards the front of the shell. 

 Ventral valve sub triangular, the margin on each side of the 

 mesial sinus broadly sinuate : mesial sinus narrow and well 

 defined quite to the beak, rapidly expanding below, and 

 produced into a broad rounded linguiform extension : beak 

 much elevated, slightly arcuate but not incurved : area high, 

 equal to about one-third the greatest width of the shell, and 

 reaching to the cardinal extremities ; foramen extending to 

 the beak, its height nearly once and a half its width at the 

 base. 



SURFACE marked by eighteen to twenty or more strong 

 simple rounded and little elevated plications, which are 

 marked by some strong concentric wrinkles of growth and 

 longitudinally striato-punctate, varying in their character, 

 dependent on the degree of exfoliation or state of preservation 

 of the surface. 



This species has usually been referred to . cuspidaius (SOWERBY); but it is much 

 more extended on the hinge-line in proportion to its height of area, and the area is 

 less extended than in SOWERBY'S figures, while the number of plications given by 

 that author are less than in our shell. There are several species of this type, com- 

 mencing in the Chemung group and extending through the Carboniferous limestones, 

 but I have not yet seen a species that can be identified with the S. cuspidatus. 



Fig. 6 a. Dorsal valve and area of a specimen of ordinary size. 



Fig. 6 ft. Ventral valve of the same. 



Geological formation and localities. In the Keokuk limestone : Keokuk, 

 Iowa ; Nauvoo and Warsaw, Illinois. 



