226 PALAEONTOLOGY OF KENTUCKY. 



limits. Anterior margin convex, the convexity increasing to the middle of 

 the postero-lateral side, thence truncated and extending in a straight line to 

 the beak, making an angle of from thirty to forty degrees with the hinge- 

 line. 



Valves depressed ; left valve regularly convex ; right valve nearly flat or 

 only slightly convex. The hinge- line is straight, having a length of from 

 two-thirds to more than three-fourths the length of the shell, and extending 

 anteriorly as far as the anterior margin. The beaks are obtuse, rounded and 

 anterior to the middle of the hinge-line. Umbo of left valve is little convex, 

 somewhat inflated, and the lines of demarcation between it and the anterior 

 and posterior wings form an angle of about one hundred and thirty degrees. 

 The wings comparatively only of medium extent ; the anterior considerably 

 smaller than the other ; both are triangular in shape. The anterior ear is sepa- 

 rated from the body by a distinct sulcus, while the posterior wing is defined by 

 the abrupt slope of the posterior side of the umbo. This slope is formed by 

 an almost rectangular deflection of the posterior margin, and has a height of 

 about one line. The terminal margins of the beaks are more or less concave, 

 becoming convex at the hinge-line. Byssal sinus broad, rounded, well defined, 

 and indicated on the ear by a sulcus extending to the extremity of the beak. 



Test is thin, marked by numerous regular alternating rays or radii, which 

 increase in number by interstitial additions, and become broader and stronger 

 towards the margins. These radii are crossed by very fine, sharp lines of 

 growth. On the wings the radiating striae are obsolete, while the lines of 

 growth are there sharper and stronger than on the body of the shell. 



The dimensions of the shells in this species are very variable ; there are very 

 large and also very small specimens. The one before me, which is one of the 

 largest ever found, measures thirty lines in height, thirty- two lines in length, 

 and has a hinge-line twenty-seven lines long ; here the length exceeds the 

 height, and the hinge-line is shorter than height or length ; but we find speci- 

 mens belonging to this species where all three dimensions are nearly equal. 



This species resembles A. pecteniforniis, but differs from it by its larger 

 wings and by its more numerous and less prominent radii. This is one of the 

 largest shells of the genus Aviculopecten, and is easily identified by its form 

 and surface-markings. All the forms heretofore placed In the two species of 

 A. sanduskyensis and A. parilis, belong to the present one. The apparent spe- 

 cific differences noted in the description of these three forms are neither con- 

 stant nor well defined, and not even sufficient to base upon them a separation 

 into different varieties. 



Formation and Locality. This species occurs in the Corniferous limestone at and around the Fall* 

 of the Ohio, in Kentucky and Indiana, where fair specimens are not very rare, though not abundant 

 either, 



