PALAEONTOLOGY OF IOWA. 489 



Interior of ventral valve marked by strong muscular im- 

 pressions, which are limited by the dental lamelles and 

 divided in the centre by a strong ridge, presenting a qua- 

 drangular area below the hinge line, the vascular impression 

 proceeding in radiating impressed lines from the base of this 

 quadrangular area : teeth strong, prominent. Dorsal valve 

 deeply concave, with strong prominent brachial processes, 

 and a median cardinal process which is sometimes bilobate. 

 Entire interior of shell punctate : striae marking the inner 

 margins of both valves, but rarely extending far beyond. 



This species is closely related to Orthis propinqua of the Upper Helderberg lime- 

 stone, to O. tulliensis of the Tully limestone, and to O. multistriatus of the Upper 

 Pentamerus limestone. It may, however, be distinguished from all these by its shor- 

 ter area and more pointed, as well as more arched, beaks, as also by the form of the 

 muscular impressions of the ventral valve and the cast of the same, as well as the 

 direction and division of the vascular impressions below. 



Fig. 4 a, b. Ventral and dorsal views of a specimen of this species. 



Fig. 4 c. Front view of the same. 



Fig. 4 d. Profile view of specimen. 



Fig. 4 e,f. Cast of the ventral valve of a young and of a full-grown individual. 



Fig. 4 g. Cast of the dorsal valve of 4/. 



Fig. 4 h. Interior of dorsal valve, showing the brachial and cardinal processes. 



Fig. 4 i. Interior of ventral valve, showing the teeth and the form of the muscular im- 

 pressions, with indistinct radiating lines below, indicating the course of the 

 vascular impressions. 



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

 the Hamilton group : at New-Buffalo, Independence, Lime creek and other 

 places in Iowa ; Rock-island ( Illinois ), etc. 



Orthis iowensis, var. furnarius. 



PLATE II. FIG. 5. 



SHELL resupinate, depressed orbicular, gibbous, with a shal- 

 low sinus in front. Ventral valve moderately convex, greatest 

 elevation near the beak, sloping gradually to the margins, 

 and becoming depressed in a broad undefined sinus. Dorsal 

 valve gibbous in the middle, arched on the umbo, with the 

 beak slightly curved over the hinge line : area narrow. 



SURFACE finely striated ; substance of the shell minutely 

 punctate. 



[IOWA SURVEY.] 62 



