LAMELL1BRANCHIATA. 93 



Pterinea flabella. 



PLATE XIV, FIGS. 1--21 ; PLATE XV. FIGS. I, 4-6, 8-10; im. PLATE I. XXXIII, FIGS II, IS 



A rinihi flabella. Conrad. Jour. Acad. Nat. Sci., Phil., vol. 3, p. 238, pi. 1l>, fig. 8. 1^1-. 



(") Va.vixem. Geol. Surv. N. Y.: Report Third Dist. 1343. 

 Compare Pterinea fascieuiata, Golbposs. Petrefacta Germanise, p. 137, pi. 120, fig. 5. 1840. 



" " (") Sandberger, Verstein, Rhein, Schichtensyat, Nassau, \>. 293, pi. 



30, fig. 7. 1S56. 

 costulala, F. A. Robmrr, Dunkkr ami vox Meyer. Palseontographica, T. 1, tig. 2. 

 Pterinea flabella. (Conrad) Hall. Pal. N. Y., vol v, pt. 1. Plates and Explanations: PI. 14. figs. 1 21 ; 



pi. 15, tigs. 1. 4-6, 8-10. Jan., 1883. 



Shell large; broad or narrow ovate, oblique, rarely erect; length from two- 

 thirds to nearly equal the height; and in some extravagant forms the height 

 is about double the length ; the greatest length below the middle ; anterior 

 and basal margins regularly curved ; the posterior margin from a little above 

 the base, follows a nearly straight line to the beak. 



Left valve more or less convex, often gibbous and arcuate. Right valve 

 flat or concave, with a little convexity on the umbo; arcuate, to correspond 

 with the curvature of the opposite valve. 



Hinge-line straight, extended upon the posterior side; length greater than 

 the length of the valve. 



Beak of left valve acute, rounded, situated near the anterior extremity of 

 the hinge, curving forwards over the hinge margin. In rare instances the 

 beak does not extend quite as far as the margin of the hinge. Umbo of the 

 left valve prominent, gibbous, limited by the sulci of the cardinal expansions, 

 and subtending an angle of from 30° to 60°. Beak of the right valve 

 depressed, rounded, not rising above the hinge; umbonal region depressed- 

 convex, gradually merging into the body of the valve, which is concave below. 



Wing large, triangular, nearly flat, varying in proportions, defined (in tin- 

 left valve) by the post-cardinal slope; margin concave; extremity acute, 

 rounded. Ear of left valve a simple rounded convex lobe or auriculation, 

 defined by a distinct, broad, rounded sulcus ; margin rounded above, sinuate at 

 its junction with the valve, forming the byssal sinus. The cardinal expan- 

 sions of the right valve are similar in dimensions to those of the opposite 

 valve; nearly in the same plane with the body of the shell, and defined 

 only by the surface stria? and the sinus. 



