144 PROCEEDINGS OF THE ACADEMY OF 



sider our shell distinct from S. glabra, were that species not known to he so 

 extremely variable. 



Locality and position. Chester and Pope Counties, Illinois. Chester Lime- 

 stone of the Lower Carboniferous series. 



LAMELLIBRANCHIA TA. 



Genus CARDIOPSIS, M. & W. 



{Cardium and o-^i;, from its resemblance to Cardium.) 



Shell equivalve, somewhat inequilateral, very slightly oblique, ovate or 

 cordiform, entirely closed ; beaks rather elevated, distinctly incurved, and 

 directed towards the anterior side ; surface marked by radiating strise or costae; 

 cardinal margin short, and rounding into the posterior border ; hinge pro- 

 vided with one or two distinct anterior teeth in each valve, near the beaks. 

 (Ligament and muscular impressions unknown.) 



In first describing the species we regard as the type of this genus, we placed 

 it provisionally in the genus Cardiomorpha of De Koninck, stating at the same 

 time that we suspected it to be generically distinct.* Prof. Hall has sinee 

 described the same species from the same locality, in the Thirteenth Annual 

 Report of the Regents of the University of New York, under the name of 

 Megambonia Lijoni, and mentions the presence of two strong anterior teeth in 

 the hinge of one valve. On clearing away the matrix from the hinge of one 

 of our specimens, we have been able to see impressions of these teeth, the 

 presence of which establishes, we think, the correctness of our suggestion, 

 that it does not properly belong to the genus Cardiomorpha. 



It is not improbable some of the species ranged by Prof. Koninck in his 

 genus, (when all their characters can be made out,) may be found congener- 

 ous with our shell; though it is manifest those he considered the typical spe- 

 cies are not, since he distinctly states that the hinge is without teeth, and 

 provided with a smooth lamina from the beaks to the posterior extremity, as 

 some of his figures show. In addition to this, all the species described by 

 him, (with one single exception, which is a transverse shell, and apparently 

 a wide departure from his typical species, as well as from ours,) are merely 

 marked by concentric strise, and show no traces of the regular radiating costa? 

 seen on our shell. 



Prof. Hall has described another species of this group, from near the same 

 horizon, under the name of Cardiomorpha ovata, (non C. ovata DeOrbigny, ) in 

 the Iowa Report, vol. i. part ii. p. 522. The radiated surface and general 

 appearance of these species give them somewhat the aspect of some species of 

 Ambonychia of Hall. They differ, however, from that group, in having no 

 posterior hinge teeth, and in being destitute of a hinge area, as well as in 

 having a shorter, and less straightened hinge. They have scarcely any rela- 

 tions to Prof. Hall's genus Megambonia, which is founded on his Pterinea ? 

 cardiformis, of the Corniferous Limestone, and belonging apparently to a 

 different family. 



Genus LEDA, Schumacher, 1817. 



Leda cdkta. Shell small, ovate, rather gibbous in the central and umbonal 

 regions ; anterior side abruptly rounded, the most prominent point being at 

 the middle ; base semi-ovate, more prominent in the antero-ventral region 

 than behind ; posterior side abruptly contracted, so as to become subangular 

 at the extremity, beaks elevated, incurved, and nearly central ; dorsal out- 

 line declining rather rapidly from the beaks, the anterior slope being convex, 

 and the posterior slightly concave, posterior umbonal slopes prominently 



* Cardiomorpha radiata, M. & W., Proceed. Acad. IS at. Sci., Phila. Oct. 1860. 



[June, 



