PALEONTOLOGY. 469 



large and strong beak, incurved; the ventral valve usually being dis- 

 tinctly concave toward the front margin, and the beak usually more or 

 less distorted and twisted. This form generally attains a considerable 

 size, occurring of a diameter of two and a half or three inches. The other 

 form is much smaller, seldom exceeding one and a half inches in its trans- 

 verse diameter; the shell is less convex, in fact never very highly rounded, 

 the cardinal area much narrower and the beak less liable to distortion. 

 These forms usually characterize different beds, and are easily recognized 

 from each other, but among them there are usually intermediate forms 

 associated, which tend to destroy the line of specific distinction, on which 

 account they are usually considered as varieties of the one species ; al- 

 though there is not the least trouble in recognizing the different types in 

 well-marked specimens, still many individuals occur which cannot be 

 satisfactorily referred to either, rendering it impossible to strictly classify 

 hem, except as one species. 



In placing this species under the genus Streptorhynckus King, I do 

 so with the belief that 5". crenistria, the shell upon which the genus was 

 iounded, is generally distinct from orthis adspectans and its congeners 

 which formed the types of Pander's genus Hemipromtes ; as, besides the 

 strong internal differences, the entire absence of a cardinal area on the 

 dorsal valve of the former shell and those of that group, and the presence 

 of a very well-developed area on that of the latter, together with the dif- 

 ference in the general form of the shell, offer good grounds for generic 

 separation. 



Genus PRODUCTUS Sowerby. 



Productus elegans. 



Plate IX, figs. 15 and 16. 



Productus. elegans N. and P., Jour. Acad. Nat. Sci. Phila., vol. iii, 2d series, p. 13, pi 



1. fig- 7. 

 Productus elegans of Authors. 

 Productus fasciculatus McChesney, New Pal. Foss., 1859, p. 38. 



Shell small, rather below a medium size, highly arcuate, and often much pro- 

 duced in older specimens; hinge-line short, frequently not more than half as long 

 as the width of the shell below. Body of the shell somewhat quadrangular in the 

 upper part, being flattened or even slightly sinuate along the median line, and also 

 flattened on the sides; beak proportionally large and obtuse, not projecting much 

 beyond the line of the hinge when viewed from above ; auriculations very small. 

 Visceral cavity proportionally small, the distance between the valves as seen when 

 the front extension of the valves is removed being not more and generally less than 

 half the width of the shell. Dorsal valve slightly concave. Surface of the shell 

 marked by strong fasciculating striae, strongest near the front, often showing some 

 stronger ones with scattered spine bases ; spines often most numerous on the sides 

 of the shell near the hinge extremities. The upper portion of the ventral and the 

 surface of the dorsal valve are marked by strong concentric wrinkles, generally dis- 

 tinct, but sometimes quite obscure. 



The Ohio specimens of the species are of very characteristic form, 

 so closely resembling those from the limestone at Chester, 111., that there 



