Contributions to Invertebrate Palaeontology. 555 



The form of the ventral valve is somewhat triangular, much 

 wider than high, the beak somewhat prominent and extended 

 beyond the line of the hinge; body of the valve ventrieose and 

 strongly arcuate, with a deep, moderately wide mesial sinus, the 

 bottom of which is occupied by a slightly elevated rib, correspond- 

 ing to the depression in the fold of the dorsal side ; from eight to 

 ten angular ribs occupy each side of the valve; cardinal area moder- 

 ately high and incurved, foramen nearly as wide as high. Surface 

 of the shell marked by strong, concentric, lamellose lines. 



Formation and Locality. — In the blue limestone layers above 

 the " fish-beds" at Delaware, Ohio. 



MOLLUSCA. 



LAMELLIBRANCHIATA. 



Genus PTERIjVEA Golrtfuss. 

 Pterinea flaliella. 



Plate XI, fig. 17. 



Avicula flabella Conrad, .Jour. Acad. Nat. Sci. Pliila., lat series, vol. 8, p. 238, 



pi. 12, fig. 8. 

 Aviculafiabella (Conrad), Vanux., Geol. Rept. 3dDist. N. Y., 1842, p. 152, fig. 3. 

 Shell obliquely subrliombic including the wings, or the body of the valves 

 alone obliquely ovate, largest below. Hinge line straight, generally pretty 

 long, especially on the posterior side, the wing on this side usually extending 

 backward as far as the posterior extremity of the body of the shell, and some- 

 times even beyond that point ; but always distinctly separated from it by a 

 broad, more or less deep, rounded sinus ; leaving the wing of a triangular 

 form, with the extremity sometimes rounded but often pointed or even mucro- 

 nate. Anterior wing on the left valve very much smaller, but still well deve- 

 loped, rounded on the margin, sloping on the cardinal border, and separated 

 from the body of the shell by a broad, often deep rounded channel, which 

 gradually widens with the growth of the shell. Body of the left valve highly 

 convex, and often with an abrupt cardinal slope much straightened beyond 

 the middle of the length. Beak large, tumid, projecting somewhat beyond the 

 cardinal border, and placed at about one-third or less than one-third of the 

 entire length of the cardinal line from the anterior extremity ; anterior border 

 of the valve rounding backward from a little below the sinus of the anterior 

 wing, and usually forming a nearly regular curve to beyond the middle of the 

 valve before beginning the upward curvature of the posterior portion. Poste- 

 rior extremity of the valve subangular at the point of greatest length. Surface 

 of the left valve marked by several strong, distinct, radiating costte, varying 



