260 PROCEEDINGS OF THE ACADEMY OF 



posterior basal margin ; Leak oblique, ratber convex, and placed very near 

 the anterior extremity of the hinge; anterior ear very srual), a little convex, 

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

 impression, rounded at the extremity, and defined in outline by a very shal- 

 low marginal sinuosity ; posterior wing large, flattened, triangular, and defined 

 by a broad, moderately deep rounded sinus, not equalling in length the most 

 prominent part of the posterior margin below the sinus in young shells ra- 

 ther acutely angular, but more obtuse in adult specimens. Surface ornamented 

 with numerous linear, radiating costse, smaller than the flattened spaces be- 

 tween, and crossed by concentric raised lines, so as to form a neat cancellated 

 style of marking, quite as distinct on the ears (particularly the posterior one) 

 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 furrows of growth. (Right valve unknown.) 



Length of the largest specimen, measuring obliquely from the most promi- 

 nent part of the posterior basal margin to the extremity of the small anterior 

 ear, 1-55 inch ; do. parallel to the hinge line, 1*41 inch ; height at right angles 

 to the hinge, 2 inches ; length of hinge and anterior ear, 1*17 inch ; length of 

 posterior ear, from the beak to its extremity, 0'91 inch. 



This rather handsome species has more the aspect of certain Upper Silurian 

 forms, such as Avicula communis, Hall, than of any carboniferous species with 

 which we are acquainted, though of course presenting well marked specific- 

 differences. 



It is a little remarkable, that all of the twenty- five or twenty-six specimens now 

 before us, are left valves, from which fact we may infer that the right valve, 

 being more fragile, was generally broken to pieces by the waves, before being 

 imbedded in the sediment. It is also probable that the right valve was less 

 convex, and more faintly marked than the other, as is usual in shells of this 

 kind. As we know nothing of the binge and muscular impressions of this 

 shell, we cannot determine whether it is a Pterinea or a Pteria. If a true 

 Pterin, and Kleins old pre-Linnaean names are to be retained, the name of our 

 shell will become Avicula morganensis. 



Locality and position. Coal Measures (below the middle), Morgan County, 

 Illinois. 



DoLABRA STEKLINGEXSIS, M. & W. 



Shell rhombic-cordate, being cordate in outline, as seen in an anterior and 

 posterior view, and obliquely rhomboidal as seen from either side. Posterior 

 margin obliquely truncated, with a long slope, which is slightly convex above 

 and faintly sinueus near the middle ; posterior basal extremity produced 

 obliquely backwards and downwards, with a more narrowly rounded or sub- 

 angular outline ; basal margin ascending forward, with a moderately convex 

 curve, and rounding up more or less gradually into the very short or almost 

 obsolete anterior side : hinge line short ; cardinal area moderately developed. 

 Beaks prominent, placed nearly over the anterior margin, strongly incurved, 

 and compressed antero-posteriorly ; umbonal ridges very prominent, sub- 

 angular, and extending from the beaks obliquely to the posterior basal ex- 

 tremity at an angle of about (38 below the horizon of the hinge, thus dividing 

 each valve into two subequal areas, of which the one behind is flattened or 

 slightly concave between the ridge and the moderately prominent postero- 

 dorsal edge, and that in front and below it convex. Surface marked with 

 concentric striae of growth. (Hinge and interior unknown.) 



Greatest length, measuring obliquely from the beaks to the posterior basal 

 extremity, 2*20 inches ; diameter at right angles to the same, 1*50 inch ; con- 

 vexity of the two valves when closed, 150 inch. 



This species is evidently related to Gjrtodonta Hindi, of Billings (see 

 Palaeonzoic Foosils of Canada, vol. 1, p. 151, fig. 131, a, b), from the same 



[July, 



