326 PALAEONTOLOGY OF ILLINOIS. 



LAMELLIBRANOHIATA. 

 Genus AVICULOPECTEN, McCoy, 1852. 



(Ann. and Mag. Nat. Hist., vii. p. 171.) 



Aviculopecten Coxanus, M. and W. 



PI. 26, fig. 6 a, 6 6. 



Aviculopecten Coxanus, Meek and Wohthen, October, 1860. Proceed. Acad. Nat. Sci., 



Philad., p. 453. 

 Comp. Aviculopecten rectilalerarius (=Avicularectilateraria, Cox), Kentucky Rep., vol. 3, 



p. 571. 



Shell (left valve) small, extremely thin, compressed, broad 

 subovate exclusive of the ears, very slightly oblique ; anterior 

 and posterior margins rounding into the base from below the 

 ears ; base regularly rounded ; hinge nearly straight, not quite 

 equaling the greatest breadth of the valves below, directed 

 very nearly at right angles to the longer axis of the shell. 

 Anterior ear of moderate size, triangular, the angle at the ex- 

 tremity being a little rounded; flattened so as to be quite dis- 

 tinct from the umbonal slope, and separated from the margin 

 below by a rather abruptly rounded sinus. Posterior ear of 

 nearly the same size as the other, but differing in being acutely 

 pointed at the extremity ; flattened, but not so distinct from 

 the umbonal slope as the anterior ear; separated from the pos- 

 terior margin by a regularly rounded, moderately deep sinus. 

 Beak compressed, scarcely projecting beyond the cardinal mar- 

 gin, and located slightly in advance of the middle of the hinge; 

 umbonal slopes diverging from the beak at an angle of about 

 77 °. Surface ornamented by small, simple, depressed and 

 rigid costse, which alternate in size, the smaller ones dying out 

 at various distances between the margin and the umbo; cross- 

 ing all of these are numerous extremely fine, closely arranged 

 concentric strise. Right valve unknown. Length, or diameter 

 from the hinge to the basal margin, 0.53 inch ; diameter, from 



