246 PALAEONTOLOGY OF NEW YORK. 



This species is distinguished by its erect form, the broad flat interspaces 

 between the radii, and the regular sharp concentric striae. 



Formation and locality. The specimen is from the higher beds of limestone 



at the Falls of the Ohio, which arc of the age of the Hamilton group. 



LlMOPTERA M ICROPTERA. 



PLATE XXIV, FIG II, PLATE XXVI. FIGs 6-8; PLATE XXVII, FIGS HO; 1'I.ATK XXVIII. Flo 1,5; PLATE 



.XXIX. FIGS 1-1 ; and PLATE X< II. FIGS. 4-'.i 



Lima macroptern, Cosrav. Annual Report N. V Geolog. Snrv., p. 117. 1838. 

 Limopte.ra \iuicruptera (Cosbad), Hall. Prelim. Notice Lam. shells, etc., pt. 'J. p. 17. Dec., 1869. 



s. A Miller, Cat. Amer. Pal. Poss. 1877. 



Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 34, fig. 14 ; 

 pi. 26, figs. 6-9 : pi. 27, tigs. 1-10 ; pi 38, figs. 4, 5 : and pi. 29, 

 figs. 1-4. Jan., 1883. 



Shell large; including the cardinal expansions, the form is subquadrate; bod} 



broadly ovate, erect or moderately oblique; wing more or less expanded; 

 height one-fourth to one-third greater than the length ; basal margin broadly 

 rounded, with the anterior and posterior sides sub-parallel. 



Valves very unequal. Left valve ranging from moderately convex to 

 very gibbous, often somewhat regularly arcuate from beak to base. Right 

 valve smaller than the opposite, in young specimens nearly flat below, 

 depressed-convex in the middle, and more convex in the umbonal region; 

 in specimens of medium size there is often a greater convexity in the upper 

 part, and in very old specimens the surface is depressed-convex and nearly 

 flat in the lower part. 



Hinge-line straight, .sometimes equalling but usually less than the length 

 of the shell. 



Beak of the left valve prominent, acute, inclined forward, rising above and 

 arching over the cardinal line, situated anterior to the middle of the body of 

 the shell. Umbonal region prominent, convex in young shells and gibbous 

 in older ones, limited anteriorly by a distinct sulcus, and posteriorly by the 

 abrupt umbonal slope. Beak of right valve acute, scarcely rising above the 

 hinge-line, defined anteriorly by the byssal sulcus, and posteriorly by the 

 abrupt depression and distinct limitation of the wing. Umbonal angle acute. 



