486 GEOLOGICAL SURVEY OF IOWA. 



BRACHIOPODA OF THE HAMILTON GROUP*. 

 Oitliis suborbicularis (n.s.). 



PLATE II. FIG. 1 a - e. 



SHELL depressed orbicular ; valves almost equally convex, 

 length and breadth about as four to five ; hinge line very 

 short. Dorsal valve regularly convex, a little flattened at the 

 cardino-lateral margins ; beak elevated and slightly incurved 

 beyond the plane of the area. Ventral valve slightly the 

 more convex, most convex near the umbo ; beak slightly 

 more elevated than the opposite, scarcely incurved : area 

 about one third as high as wide : foramen large ; width and 

 height about equal. 



SURFACE finely striated ; striae increasing by implantation, 

 gradually curving outward towards the margin, and bending 

 upward to the hinge line, crossed by fine concentric striae, 

 and sometimes by imbricating lines of growth. 



Interior marked near the margin by strong striae which 

 are faintly visible half way to the beak ; the entire surface 

 finely punctate. Ventral valve with strong cardinal teeth, 

 and muscular impressions limited by the dental lamellae. 

 Dorsal valve with strong brachial processes and a small car- 

 dinal process. 



This species bears some resemblance externally to O. vanuxemi of the Hamilton 

 group, as it occurs in its more circular forms in New- York; but is proportionally 

 broader, and the valves more equally convex. In its muscular and vascular impres- 

 sions it is conspicuously distinct, as well as in the cardinal process and strongly 

 marked ridge down the centre of the dorsal valve. It bears, also, much resemblance 

 to a species of the Lower Helderberg limestone, but careful comparisons show it to 

 be distinct. 



The specimen figured is somewhat compressed towards the margins, so that a 

 profile view does not present the true form of the shell. 



Fig. 1 a, b. Dorsal and ventral views of an entire individual. 



Fig. 1 c. Cardinal view of the same enlarged. 



Fig 1 d. Interior of a dorsal valve, showing the brachial and cardinal processes. 



Fig. 1 e. Interior of a ventral valve of a small individual, showing the cardinal teeth. 



* For generic descriptions of the Brachiopoda, see pages following the specific de- 

 scriptions at the end of the volume. 



