488 GEOLOGICAL SURVEY OF IOWA. 



Tig. 2 a, b, c. Ventral, dorsal and profile views of a specimen of the usual form of the 



species in its more northwestern localities. 

 Fig. 2 d. Cardinal view, enlarged, to show area and foramen. 

 Fig. 2 e,f. Dorsal and ventral views of a specimen from the same geological position in 



Missouri. 



Fig. 2 g, h. Interior of ventral and dorsal valves. 

 Fig. 2 i. Enlargement of striae. 



Fig. 3 a, b. Ventral and profile views of a specimen of medium size, from the shales of 

 the Hamilton group in New-York. 



Fig. 3 c, d. Interior of ventral and dorsal valves from New-York specimens; the ventral 

 valve showing some slight differences in form and divergence of the teeth 

 from 2 g. 



Fig. 3 e. Interior of a large ventral valve from the Hamilton group of New-York, show- 

 ing the vascular and muscular impressions : the teeth are broken. 



Fig. 3 f. Enlargement of the striae, showing the punctate surface and more distinct tu- 

 bular openings. 



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

 Hamilton group : near Davenport and New-Buffalo, Iowa ; Rock-island, 

 Illinois, and several localities in Missouri ; the Hamilton group in Central 

 and Western New- York. 



Orthis iowensis. 



PLATE II. FIG. 4 a - i. 



SHELL resupinate, transversely oval or subglobose, with a 

 deep sinus in front; hinge line less than two-thirds the 

 greatest width of the shell. Ventral valve much the less 

 convex, greatest convexity near the beak, and flattened to- 

 wards the margins, with a deep sinus from the middle to the 

 base of the valve ; beak elevated slightly above the opposite, 

 pointed, not incurved. Dorsal valve extremely gibbous, 

 greatest convexity about the centre of the valve, and slop- 

 ing abruptly to the sides ; umbo arched ; beak prominent, 

 slightly incurved over and projecting beyond the hinge line. 

 Area small ; foramen narrow. 



SURFACE marked by fine closely arranged radiating tubular 

 striae which increase by bifurcation and interstitial addition, 

 and are crossed by fine concentric striae and a few imbricating 

 lines of growth; radiating. striae presenting numerous tubu- 

 lar openings upon the surface, and marked by fine pores or 

 punctse over the entire surface. 



