PALEONTOLOGY. 449 



MOLXUSCA. 



LAMELLIBRANCHIATA. 



Genus PTERINEA Goldfuss. 



' Pterinea flabella. < 



Plate VII, fig. 17. 



Avicula flabella Conrad, Jour. Acad, Nat. Sci. Phila., 1st series, vol. 8, p. 238, pi. 12, 



fig. 8. 

 Avicula flabella (Conrad), Vanux., Geol. Rept. 3d Dist. N. Y., 1842, p. 152, fig. 3. 



Shell obliquely subrhombic 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 sometimes even be- 

 yond 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 mucronate. Anterior wing on the 

 left valve much smaller, but still well developed, 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 valves highly convex, and often with an abrupt cardinal slope much straight- 

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

 yond 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 be- 

 fore beginning the upward curvature of the posterior portion. Posterior extremity 

 of the valve subangular at the point of greatest length. Surface of the left valve 

 marked by several strong, distinct, radiating costee, varying in number on different 

 individuals but usually ten or twelve ; those making the centre of the valve and on 

 the umbonal slope being nearly straight in their direction from the beak to the base 

 of the shell ; while those nearer the anterior end become more and more curved in 

 their direction as they approach the margin. Prom three to five intermediate costse 

 occupy the usually slightly concave but often flattened interspaces. The entire sur- 

 face of the valve is often marked by more or less strongly marked concentric lines 

 of growth, which in crossing the stronger radii often form lamellose projections on 

 their surface when perfectly preserved, but are usually represented by small knotty 

 prominences as commonly seen. The posterior wing is often marked by indistinct 

 radiating lines, though not uncommonly these are entirely obsolete, and the con- 

 centric lines are strongly marked. The right valve of this shell is very slightly 

 concave, proportionately smaller than the left, with the radiating lines much sub- 

 dued, and the concentric lines not so elevated or knotty. 



The specimens of this species observed from the rocks above the 

 " Bone-bed," in Ohio, have been left valves mostly. I have a recollection, 

 however, of having seen one slab in the Ohio State Collection, at Colum- 

 bus, which contained the impression of one right and one left valve, pos- 

 sessing the usual features of the species common in the Hamilton rocks 

 of New York; and which was said to be from layers above the "Bone- 

 bed." The specimen figured on Plate XII, fig. 17, is from this horizon, 

 and presents all the features common to the New York Hamilton forms, 

 including the great gibbosity of the left valve. In the Upper Helder- 

 29 G. O. 



