530 PALiE ONTOLOGY OF ILLINOIS. 



narrowly rounded below ; anterior side very short, sloping abruptly 

 from the beaks above, and abruptly rounded beueatb ; binge line very 

 straight, rather long, but shorter than the base. Beaks depressed upon 

 a line with the dorsal outline, and located very near the anterior margin ; 

 umbonal ridge prominent and distinctly angular from the beaks to the 

 posterior basal extremity. Surface of internal cast, showing faint traces 

 of two or three distaut, very obscure, concentric ridges, or undulations. 



Length 0.52 inch, hight 0.20 inch, convexity 0.16 inch. 



The most marked peculiarities of this species are its oblong form, 

 straight and parallel cardinal and ventral margins, and distinctly angu- 

 lar umbonal ridge. Its anterior muscular impression seems not to be as 

 distinct as usual in the genus Pleuropliorus, but this may be due to a 

 defect in our specimen, which is an internal cast. All we know of the 

 hinge is an impression of a long, linear posterior lateral tooth, parallel 

 to the cardinal margin, and most distinct behind. This tooth appears 

 to have been double in the left valve, for the reception of a similar elon- 

 gated tooth in the right. 



Position and locality : Wabash cut-off, near New Harmony, Indiana ; 

 Upper Coal Measures. 



Geis-us OAEBOKAECA, Meek and Worthen. 



Genus Carbonarca, Meek and Woethen, 1870. Proc. Acad. Nat. Sei., Pliilad., page 39. 



Shell (as determined from internal casts) equi valve, inequilateral, very 

 convex, transversely oblong or oval ; umbOnes gibbous, prominent, and 

 strongly incurved with subangular or prominent posterior slopes ; valves 

 closed all around, with smooth margins ; ligament external ; cardinal 

 margin a little arched, with, at the anterior extremity in each valve, 

 two rather oblique comparatively stout teeth, and extending along its 

 entire length from immediately behind these, a row of minute, inter- 

 locking teeth or crenulations, as in Area. 



This genus seems to belong to the Arcidce, near Isoarca. It differs, 

 however, very decidedly from that genus, in having, in addition to the 

 small interlocking crenulations along the whole length of the hinge, two 

 well developed and independent larger teeth at the anterior end of the 

 hinge. The specimens seen are all internal casts, but an impression of 

 the hinge of a right valve, in the matrix, shows its characters very 

 clearly. There is no gradation from the series of minute teeth into the 

 two large ones at the anterior end of the hinge, the first of the smaller 

 series immediately behind the two larger ones being as minute as any 

 of those farther back ; so that the contrast between the two sets of teeth 

 is well marked and abrupt. The hinge margin was doubtless provided 

 ■with a cardinal area, but as we only have internal casts, it has not yet 

 been seen. 



