454 PROCEEDINGS OF THE ACADEMY OP 



angles to the vertical axis of the shell. Left ? valve compressed ; anterior ear 

 flat, having the form of an inequilateral triangle, the anterior side of which is 

 much shorter than either of the others ; rather acutely angular at the ex- 

 tremity, and separated from the margin below by a nearly rectangular notch ; 

 posterior ear apparently nearly of the same size as the other, compressed, and 

 separated from the margin below by a rounded, moderately deep sinus : beak, 

 small, compressed, and not extending above the hinge ; surface marked by 

 rather small, obscure, depressed, radiating costae, which are crossed by a few 

 faint marks of growth. 



Greatest breadth parallel to the hinge, a little below the middle, 3.39 

 inches ; diameter at right angles to the same, 3 inches ; length of hinge 3 

 inches. 



A fragment, probably of the other valve of the same species, embedded in 

 the same mass with that described above, has much more prominent and 

 more angular costae, crossed by smaller, closely arranged concentric lines, so 

 as to give them a slightly crenulated appearance. 



Named in honor of Prof. L. De Koninck, the distinguished palaeontologist of 

 Liege. 



Locality and position. Alpine, Iowa. Lower Coal Measures. 



Aviculopegten interlineatus. Shell rather small, compressed, broad ovate 

 or subcircular, exclusive of the ears ; length and breadth nearly equal ; hinge 

 straight, scarcely equalling the greatest breadth, ranging at right angles 

 to the axis of the shell. Left valve with base regularly rounded ; posterior 

 and anterior margins prominent near the middle, and rounding to the pallial 

 margin, rather distinctly sinuous tinder the ears ; anterior ear triangular, 

 flattened, so as to be quite distinct from the umbonal slope ; posterior ear 

 somewhat larger than the other, compressed, triangular, the hinge side being 

 longer than either of the others, rather acutely angular at the extremity ; 

 beak a little nearer the anterior than the posterior side, not oblique ; surface 

 ornamented by about fifteen regular, very prominent, slender, concentric 

 costae, separated by spaces some four or five times their own breadth ; spaces 

 between the costae occupied by numerous very fine, regular, closely arranged 

 concentric stria?, which are crossed by very obscure traces of radiating ribs. 



Diameter, from hinge to pallial border, 0-60 inch ; breadth, 0-62 inch ; con- 

 vexity. 0*12 inch. (Right valve unknown.) 



Locality and position. Lasalle, Illinois. Upper Coal Measures. 



Avicclopecten amplus. Shell large, distinctly inequi valve ; height and 

 breadth about equal ; ventral margin regularly rounded ; posterior and anterior 

 sides rounding to the base ; hinge line straight, less than the breadth of the 

 valves. Left valve convex, particularly in the umbonal region ; posterior 

 ear very short, obtusely siibangular, and sometimes defined by an obscure 

 sinus in the margin below ; anterior ear separated from the swell of the umbo by 

 a more or less deeply rounded sulcus, (form unknown ;) beak rather gibbous, 

 incurved, slightly oblique, and extending a little above the hinge. Surface 

 ornamented by distinct, narrow, rather elevated, obscurely subnodose, 

 radiating plications, separated by spaces five to seven times their breadth, and 

 crossed by fine, indistinct lines and wrinkles of growth ; between the costa? 

 there are also fine radiating stria?, one or two of which are often larger than 

 the others. Right valve flat, or a little concave ; posterior ear short, nearly 

 rectangular, the margin below it being faintly sinuous ; anterior ear also short, 

 and separated from the margin below by a very deep, narrow, somewhat arched 

 and angular sinus ; beak entirely obsolete ; surface smooth, or only having 

 indistinct marks of growth, and a few faint radiating lines. 



The hinge area of both valves is rather broad, and marked parallel to its 

 margin by fine grooves. In the left valve it is slightly inclined towards the 

 right side, over that of the other valve, in which it is also inclined j backwards 



roct. 



