FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 225 



Aviculopecten pecteniformis. CONRAD. 



Plate III., figure 1. 



Avicula pecteniformis, Conrad. Jour. Acad. Nat. Sci. Phil. 1842. 

 Aviculopecten pecteniformis, Hall. Pal. of N. Y., Vol, 5, pt. 1, p. 4 1883. 



Shell sub-ovate, oblique to the hinge-line ; length about equal to height ; 

 anterior and basal margins regularly rounded, while the posterior margin of 

 the body forms an almost straight line to beak. Left valve almost flat, only 

 moderately convex in the umbonal region. The hinge-line is straight, and of 

 about two-thirds the length of shell, or even somewhat more. Umbo moder 

 ately elevated, abruptly sloping to the wings, thus forming a demarcation-line 

 between the ears and the body of valve. These umbonal lines form an angle of 

 a little more than ninety degrees. The wings have a triangular form and are 

 of medium size, the anterior one having but little more than half the area of 

 posterior wing. The beak is of moderate size and pointed ; it is located ante- 

 rior to the middle of the cardinal line. The terminal margins of the wings 

 are more or less concave. The interior of this shell and its right valve are not 

 known. 



Test is thin and ornamented by about thirty-five sharp and strong radiating 

 striae, with intermediate finer ones, which are crossed by fine, imbricating, 

 concentric lines of growth, some of which are more prominent, and divide the 

 surface in different concentric zones. 



On the wings the radiating striae are obsolete, and even the lines of growth 

 are there scarcely visible. This species is variable in its dimensions in differ- 

 ent specimens. The one before me measures twenty-five lines in height, by 

 two inches in length, and with a hinge-line of one inch and one-half. 



This species has great resemblance to A. princeps, from which it differs, 

 however, by its smaller ears, and by its coarser or stronger principal radii, 

 while the intermediate finer striae are so fine as being scarcely visible to the 

 naked eye, which makes the interspaces between the principal striae appear to 

 have three times the width of these radii. 



Formation and Locality. Occurs in the Corniferous limestone of the Devonian formation at and 

 around the Falls of the Ohio, in Kentucky and Indiana. It is a rather rare species, especially in fair or 

 well preserved specimens. 



Aviculopecten princeps. CONBAD. 



Monotis jtrinceps, Conrad. Ann. Kep. N. Y. Geol. Surv. 1838. 

 Avicula parilis, Conrad. Proc. Acad. Nat. Sci. Phil., Vol. 81842. 

 Aviculopecten princeps, Hall. Pal. N. Y., Vol. 5, pt. 1 1883. 

 Aviculopecten sanduskyensis, Hall. Pal. N. Y., Vol. 5, pt. 1 1883. 



Shell large, obliquely broad-ovate ; axis inclined more than sixty degrees to 

 the hinge-line ; length and height nearly equal, varying within moderate 



GEOL. SUK. 29 



