Contributions to Invertebrate Palaeontology. 581 



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 con- 

 sidered as varieties of the one species ; although there is not the 

 least trouble in recognizing the different types in well-marked 

 specimens, still many individuals occur which cannot be satisfac- 

 torily referred to either, rendering it impossible to strictly classify 

 them, except as one species. 



In placing this species under the genus Streptorhynchus King, I 

 do so with the belief that S. crenistria, the shell upon which the 

 genus was founded, is generally distinct from Orthis adsjiectans 

 and its congeners, which formed the types of Pander's genus 

 Hemipronites ; 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 difference in the general 

 form of the shell, offer good grounds for generic separation. 



Genus PRODUCTUS Sowerby. 



Prodiictus elegans. 



Plate XIII, figs. 15 and 16. 



ProductHS 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 

 produced 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 quad- 

 rangular 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 fascicu- 

 lating strial, 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. 



