452 PROCEEDINGS OF THE ACADEMY OF 



greatest convexity near the middle of the valves, which are equally convex. 

 Lateral margins rather narrowly rounded in the middle, thence converging 

 with a slightly convex outline to the middle of the front, which is faintly 

 subtruncate ; from the most prominent part of the lateral margins, they 

 converge to the beaks at an angle of about 97. Both valves without a distinct 

 mesial fold or sinus, though they are each sometimes marked by a narrow, 

 scarcely perceptible flattening along the middle. Beak of ventral valve small, 

 slender, and closely incurved upon that of the opposite valve, which is little 

 less prominent. Surface marked by fine lines of growth, and small, rather 

 obscure, concentric wrinkles. Internal spiral appendages, each making about 

 twelve turns. 



Breadth of a rather wide specimen - 82 inch ; length of do. 0*75 inch ; 

 convexity 0*52 inch. 



Locality and position. Near Warsaw, Illinois. Keokuk Limestone of Lower 

 Caboniferous series. 



CONCHIFERA. 



Genus PECTEN, Linnseus. 



Pecten tenuilineatus. Shell rather small, broad ovate or subcircular, ven- 

 tral border regularly rounded ; posterior edge forming a broad, gentle curve 

 along the middle and below, and intersecting the hinge above at an angle of 

 about 110 ; anterior margin rounding regularly into the base from near the 

 middle ; hinge short, or at out half the length of the valves, from the posterior to 

 the anterior side. Right valve compressed ; posterior ear nearly obsolete, flat, 

 and not separated from the posterior margin by a sinus ; anterior ear larger 

 than the other, rounded at the extremity, and separated from the margin 

 below by a deep, acutely angular sinus, from the extremity of which there is 

 a sulcus extending obliquely upward in the direction of the umbo ; beak 

 rather compressed, located slightly behind the middle of the hinge, and not 

 extending above its margin ; surface apparently smooth, but showing under 

 a good magnifier extremely fine, closely arranged concentric stria?, which be- 

 come much stronger on the anterior ear. (Left valve unknown.) 



Height, from ventral margin to the hinge, - 56 inch ; breadth, from an- 

 terior to posterior side, 0-53 inch; length of hinge, 0-27 inch; convexity of 

 right valve, - 08 inch. 



Locality and postion. South line of Clinton county, Illinois. Upper Coal 

 Measures ; associated with Modiola tenuiradiata (=Mytilus tenuiradiatus, 

 Swallow), and Avicula ? longispina, (= Gervillia longispina, Cox,) or a closely 

 allied species. 



Genus AVICULOPECTEN, McCoy. 



Aviculopecten Oweni. Shell small, truncato-subcircular ; base regularly 

 rounded, posterior margin rounding from above the middle to the ventral 

 border ; anterior side rounded below ; hinge equalling about two-thirds the 

 diameter of the valves, from the posterior to the anterior side. Left valve 

 moderately convex ; anterior ear compressed, separated from the swell of the 

 umbo by a rounded shallow depression, and defined by a shallow sinuosity of 

 the anterior margin; having the form of an equilateral triangle, the anterior 

 side of which is shorter than either of the others, very slightly rounded at 

 the nearly rectangular extremity ; posterior ear a little smaller than the 

 other, compressed, very short, and not separated from the margin below by a 

 distinct sinus, terminating in an obtuse angle of more than 100 ; umbo 

 rather convex, located apparently a little behind the middle of the hinge, and 

 scarcely extending beyond its margin ; surface ornamented by numerous fine , 

 closely arranged, radiating striae, which increase by implantation, and are 

 crossed by a few irregular concentric marks of growth. 



Diameter, from ventral margin to hinge, 0*48 inch ; do. from the anterior 



[Oct. 



