FOSSILS OF THE COAL MEASURES. 577 



liquely sub-ovate, moderately convex, very thin ; anterior 

 and basal margins forming an obliquely descending semi- 

 oval or semicircular curve from the anterior ear to the pos- 

 terior margin, which is prominently and rather narrowly 

 rounded; hinge line somewhat less than the length of the 

 shell, and ranging at an angle of about forty-five degrees 

 above a line drawn from the beak to the most prominent 

 part of the posterior basal margin; beak oblique, rather 

 convex, and placed very nearly over the anterior extremity 

 of the hinge; anterior ear very small, a little convex, but 

 separated from the swell of the umbo by an oblique, shallow, 

 rounded sulcus or impression, rounded at the extremity, 

 and defined in outline by a very shallow marginal sinu- 

 osity; posterior wing large, flattened, triangular, and de- 

 fined in outline by a broad, moderately deep, rounded sinus, 

 not equaling in length the most prominent part of the pos- 

 terior margin below the sinus; in young shells rather acutely 

 angular, but more obtuse in adult specimens. Surface orna- 

 mented by numerous linear, radiating costae, smaller than 

 the flattened spaces between, and crossed by raised concen- 

 tric lines, so as to form a neat cancellated style of marking, 

 quite as distinct on the posterior wing as on the body of 

 the valve ; radiating costre increasing by intercalation, the 

 intermediate ones dying out at various distances between 

 the free margin and the beak, all more or less interrupted 

 at various intervals by irregular, shallow concentric fur- 

 rows of growth. (Right valve unknown.) 



Length of the largest specimen, measuring obliquely from 

 the most prominent part of the posterior basal margin to the 

 extremity of the small anterior ear, 1.55 inches; do., par- 

 allel to the hinge line, 1.41 inches; hight at right angles to 

 the hinge, 2 inches; length of hinge and anterior ear, 1.17 

 inches; length of posterior ear, from the beak to its extrem- 

 ity, 0.91 inch. 



This rather handsome species has more the aspect of certain Upper 



Silurian forms, such as Ariculn com in mils, Hall, than of any other Car- 



