INVERTEBRATES. 257 



Aviculopecten amplus, M. and "W. 



PI. 18, Fig. 4 a, 4 6, 4 c. 



Aviculopecten amplus, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 454. 



Shell large, subcircular, rather thin, not oblique; length 

 and breadth about equal; ventral and lateral margins regularly 

 rounded; hinge line straight, less than the breadth of the 

 shell. Ligament area broad, distinctly striated longitudinally; 

 that of the right valve inclining back from the hinge, and the 

 other overhanging it. Muscular impression large, obliquely 

 subrhombic, concave in outline above, and convex below ; loca- 

 ted above and behind the middle. 



Left valve convex, particularly in the umbonal region ; pos- 

 terior ear very short, nearly rectangular, sometimes very 

 slightly sinuous on the margin, not very distinct from the 

 swell of the umbo ; anterior ear subtrigonal, angular at the 

 extremity, and defined by a deep subangular sinus in the mar- 

 gin below — separated from the swell of the umbo by a more or 

 less distinct, rounded sulcus or depression. Beak gibbous, in- 

 curved, and extended slightly beyond the hinge margin. Sur- 

 face ornamented by distinct, narrow, obscurely subnodose, radi- 

 ating costae, separated by spaces from five to seven times their 

 own breadth, in which a few much smaller costae or radiating 

 lines are sometimes seen ; crossing the whole are numerous 

 small, irregular concentric wrinkles, and rather obscure lines 

 of growth. 



Right valve flat, or concave ; posterior ear very small, some- 

 times obsolete, and in other specimens separated from the mar 

 gin below by a very slight sinuosity; anterior ear oblong, 

 rounded at the extremity, and separated from the margin be- 

 low by a very profound, acutely angular byssal sinus; umbo 

 entirely obsolete. Surface nearly smooth, or marked with 

 very obscure concentric wrinkles, and faint traces of radiating 



33 Sept. 12, 1866. 



