232 PALAEONTOLOGY OF ILLINOIS. 



posterior ear somewhat larger than the other, flat, and termi- 

 nating in an angle of 45° at the extremity — separated from 

 the margin below by a wide, deep, subangular sinus, beak 

 compressed and located slightly in advance of the middle of 

 the hinge. Surface (left valve) ornamented by eighty to ninety 

 small, nearly equal, radiating costse, which increase by im- 

 plantation, and about equal the breadth of the depressions 

 between ; costae crossed by numerous, very regular, undulating, 

 and distinctly imbricating, concentric laminae of growth, which 

 are very closely arranged and sharply elevated on the ears, 

 where the radiating ribs are not developed. (Right valve 

 unknown.) Length from hinge to base, about 2.24 inches; 

 breadth from anterior to posterior side, about 2.27 inches; 

 length of hinge, 1.68 inches; number of costae in 0.40 inch at 

 the ventral margin, 10 to 12; number of concentric marks in 

 same space, about 18. 



In its peculiarly delicate surface markings, this shell closely resembles A. 

 coelatus, of McCoy', as figured in his Palaeozoic Fossils, pi. 3 E, Jig. 5 and 5 a. 

 It differs, however, materially in form, its lateral margins being more abruptly 

 rounded, and the sinuses under the ears deeper and more angular. It also dif- 

 fers in being entirely destitute of any traces of radiating costse on the ears. 



Loality and position: Burlington, Iowa; upper part of Burlington group of 

 the Subcarboniferous series. 



