PALEONTOLOGY OF IOWA. 521 



This species has only been found as casts, and is apparently identical with one 

 from the same locality, figured by Dr. OWEN as Spirifer euruteines ; but the original 

 of that species is, I believe, from the limestone of the Falls of the Ohio, and the 

 species from Pine creek here figured is quite distinct from those of the limestone. 

 It becomes, therefore, necessary to separate this one under a distinct designation. 



The species most nearly approaching this, are S. textus and S. carteri of the Yellow 

 sandstone of the Chemung group in Ohio; but the differences are well marked. 



Fig. 7 a. Dorsal valve of a large individual. 



Fig 7 b. Ventral valve of the same. 



Fig. 7 c. Profile view. 



Fig. 7 d. Cardinal view, showing height of area and foramen. 



Geological formation and locality. In yellow sandstone of the age of the 

 Chemung group : near the mouth of Pine creek. 



Spirifer subrotundatus ( n. s.). 



PLATE VII. FIG. 8 a, b. 



SHELL subglobose ; extremities very conspicuously rounded ; 

 hinge line much less than the width of the shell. Ventral 

 valve very gibbous, or subventricose ; umbo large ; beak in- 

 curved ; cardinal margins much incurved ; mesial sinus 

 shallow, not defined, and covered by dichotomizing plications 

 like those on the other parts of the shell : area high, com- 

 paratively narrow, diminished by the incurved cardinal 

 margins of the valve ; foramen large. Dorsal valve the less 

 convex, gibbous in the middle, and gradually and uniformly 

 curving towards the margins ; mesial fold not defined and 

 scarcely elevated above, becoming more elevated but still 

 undefined towards the base. 



SURFACE covered by fine radiating, dichotomizing striae, 

 about ten or twelve of which occupy the mesial fold and 

 sinus, and as many as thirty on either side, the plications 

 becoming inconspicuous towards the margins of the casts. 



This species is sufficiently characteristic in form and surface markings to be rea- 

 dily distinguished from all others known to me in the rocks of the West. 

 Fig. 8 a. Cast of the dorsal valve of this species. 



Fig. 8 b. Cast of the ventral valve, showing the narrow, elongate muscular impressions 

 below the beak. 



Geological formation and locality. In yellow sandstone of the age of the 

 Chemung group, and overlying the Oolitic limestone : Burlington, Iowa. 

 [ IOWA SURVEY.] 66 



