BBACHIOPODS. 73 



Strophomena planumbona (HALL). 



Plate xxxix, fig 3 . 

 Leptcena planumbona Hall, 1847: Pal. New York, vol. I, p. 112, pi. xxx B, 



figs. 4a-d. 

 Strophomena planumbona Hall, 1859: 12th Reg. Rep. State Cab. New 



York, p. 70. 



Strophomena planumbona Hall, 1862: Geol. Sur. Wisconsin, p. 54, fig. 7. 

 Strophomena planumbona Meek, 1873 t Geol. Snr. Ohio, Pal., vol. I, p. 79, 



pi. vi, figs. 3a-h. 

 Strophomena planumbona White, 1881 : Geol. Sur. Indiana, 10th Ann. 



Rept. , p. 480, pi. ii, figs. 13-14. 



Shell rather small, or scarcely attaining a medium size, 

 concavo-convex, semi-oval, or more than semicircular in outline; 

 hinge-line generally a little longer than the breadth of the 

 valves at any point farther forward; lateral extremities, in 

 most examples, somewhat less than rectangular, or sometimes 

 rather acute, more or less compressed and deflected ; lateral 

 margins a little contracted posteriorly, and rounding to the 

 front, which forms a regular semicircular curve. Dorsal valve 

 flat in the umbonal region, and rather strongly and evenly con- 

 vex in the central and interior regions, from which it rounds 

 off abruptly to the front and lateral margins ; beak very small, 

 or not distinct from the edge of the narrow or sublinear area, 

 which is inclined nearly directly backward, but not incurved. 

 Interior showing the cardinal process to be small, depressed, 

 divided to its base into two diverging tooth-like parts, a little 

 flattened on their posterior faces, and directed very obliquely 

 forward and outward ; socket ridges short and oblique ; mesial 

 ridge low, extending but a little distance forward; while the 

 space between it and the socket ridge, on each side, is occupied 

 by a moderately distinct muscular scar. 



Ventral valve broadly and rather deeply concave in the 

 central and anterior regions, and sharply convex at the beak, 

 which is very small, abruptly pointed, scarcely projecting be- 

 yond the edge of the area, and usually minutely perforated ; area 

 moderately high, extending the whole length of the hinge, gen- 

 erally but little sloping laterally, flattened and inclined more or 

 less backward ; foramen closed by a prominent, rounded pseu- 

 dodeltidium that is transversely striated, and rather broadly 

 G-6 



