FOSSILS OF THE HUDSON RIVER GROUP. 79 



tended into a rather long, acute point, when perfect, forming nearly one- 

 third of the length of the hinge, measured from the point of the beak ; 

 posterior wing large, rather obtusely pointed, and extending as far as the 

 body of the shell below ; body of the shell oblique, a line drawn from the 

 beak to the center of the base forming an angle with the posterior hinge- 

 line of about sixty-five or seventj^ degrees ; posterior margin of the shell 

 broadly and roundly, but not deepl}^, excavated between the posterior 

 wing and the postero-basal extremity of the shell; basal margin rather 

 sharply rounded; anterior margin obliquely sloping from the hinge-line, 

 being nearly parallel with the body of the shell ; very slightly excavated 

 below the anterior alation. Left valve strongly convex when not com- 

 pressed, prominent and rounded in the center, but flattened and slightly 

 concave toward the alations; beak small, extending but little above the 

 hinge-line ; flattened or depressed convex on the umbo. Right valve 

 concave, the concavity not exceeding one-half the convexity of the oppo- 

 site valve, and usually somewhat shorter on the basal portion. 



Surface of the convex valve marked by regular, concentric, lamellose 

 lines, the edges of which are sharply elevated when well preserved, giv- 

 ing an exceedingly roughened character to the surface. In the degree of 

 this latter feature, as also in the relative distance of the lines, there is 

 considerable variation in different individuals. Surface of the concave 

 valve distinctly lamellose, but the precise features have not been very 

 clearly determined, as no very good specimens of this valve have been 

 examined. 



Formation and locality : Specimens of this species are not uncommon in the upper 

 part of the beds of the Hudson River group, as seen at Cincinnati, Ohio, which 

 do not appear to differ in any important feature from specimens found in the 

 same formation in New York. The species is probably found at most localities of the 

 formation, or will be on careful examination, as it is a form having a very extended 

 geographical range. 



Genus AMBONYCHIA, Hall. 



(Pal. N. Y., Vol. I., p. 163 ; 1847.) 



Ambonychia radiata. 



Plate 2, fig. 2. 

 Ambonychia radiala, Hall ; Pal. N. Y., Vol. L, p. 292, pi. 80, fig. 4. 

 Ambonychia radiata, Hall ; 12th Rept. State Cab., p. 110. 

 Ambonychia radiata, Hall; Geol. Rept. Wis., p. 54. 



Shell small to medium sized, varying in outline from sub-quadrangular, 

 with a rounded base, to acutely ovate, according to the degree of obliquity 



