FOSSILS OF THK COAL M FASI ' I: |-;s. 5S7 



ascends very abruptly and a little obliquely forward, with 

 a slightly convex outline, to the lower part of the anterior 

 hiatus. Anterior gaping edge truncated, with a slightly 

 convex outline and forward slope from immediately in front 

 of the beakfl, and defined, or separated from the hody of the 

 shell hy a faint suleus. starting from the immediate front 

 of the beak, and curving downward so as to intersect , the 

 margin at the base of the hiatus, which (supposing it to he, 

 <Mjiially developed in the right valve) is about twice as lonu 

 as wide, and of a broad, lance-ovate form, most angular 

 at the base. Hinge line scarcely more than one-fourth the 

 greatest length of the shell, measuring obliquely from the 

 the anterior extremity to the posterior basal margin, and 

 ranging at an angle of about forty five degrees to the longer 

 axis of the valves. Beaks very small, incurved, and de- 

 pressed almost to the cardinal margin; located nearly over 

 the anterior edge?. Surface with concentric stria- obscure; 

 radiating cost;c of the posterior dorsal region very faintly 

 marked, and broader than the slight furrows between, while 

 very obscure traces of fine longitudinal striae may be seen 

 on some of them; radiating costse of the anterior tine, and 

 rather sharply defined on the gaping edge, hack of which a 

 few larger, obscure ribs maybe seen, the posterior one of 

 'which is largei- than the others, and curves down from the 

 anterior side of the beak so as to intersect the margin of 



o 



the valve a little below the lower end of the hiatus. 



Length, 2 inches; breadth, measuring at right angles to 

 the greatest length, 1.48 inch; convexity of the left valve, 

 0.50 inch; length of anterior hiatus, 0.6'3 inch; breadth of 

 do. in same, ().!(> inch. 



This shell dillers so widely from nil others known to us from our 

 Carboniferous rocks, that a comparison is unnecessary. Indeed, we 

 know ot no other form liable to be confounded with it, from rocks of 

 any a&c. 



L<x-nlihl aMdpOgitionWsuipeean creek, (irundy county, Illinois; from 

 the, lower part of the Coal .Measures. 



