712 GEOLOGICAL SURVEY OF IOWA. 



concave, continued to the extremities of the hinge-line, 

 vertically striated : foramen large, forming an equilateral 

 triangle. 



SURFACE marked by from eight to ten simple abruptly 

 elevated plications which are equal to the spaces between, 

 concentrically marked by strong imbricating lamellose lines 

 of growth, and, on well-preserved specimens, by finer ra- 

 diating and concentric striaB. 



Fig. 1 a, b. Profile and dorsal views of a large individual. 



Geological formation and localities. In the Coal measures of Ohio, Mary- 

 land, Iowa, etc. 



Productus costatus, var. 



PLATE XXVIII. FIG. 3 a, b, and 4 a, b, c. 



Producta costata : J. D. C.SOWERBY, 1827, Min. Conchology, Vol. vi, pa. 115, pi. 560, f.l. 

 For synonomy and references, see DE KONINCK, Mon. of Productus and Chonetes, p. 92. 



SHELL somewhat hemispheric, transverse, about as long as 

 wide. Ventral valve strongly arcuate, ventricose in the mid- 

 dle on each side, with a broad shallow depression down the 

 centre which does not extend to the beak, constricted at the 

 cardinal extremities below the vault, and extended in short 

 sometimes recurved auricles ; beak incurving a little beyond 

 the hinge-line ; sides rounded, compressed, the front usually 

 somewhat sinuate. Dorsal valve regularly concave in the 

 middle, flattened or broadly grooved towards the hinge ex- 

 tremities. 



SURFACE of ventral valve marked by numerous rounded 

 costas, many of which frequently dichotomise on the upper 

 part of the shell, and often coalesce and again subdivide 

 below : cardinal margin marked by two or three spines on 

 each side (two often quite conspicuous on each of the ears), 

 and a row of spines upon the fold just within the constric- 

 tion bounding the ears, extending from near the beak in a 

 curving line towards the base of the shell.' Entire surface 

 of the valve marked at irregular intervals by the bases of 



