330 PALAEONTOLOGY OF ILLINOIS. 



breadth nearly equal; hinge straight, about equaling the 

 greatest breadth of the valve below, ranging at right angles to 

 the vertical axis of the shell. Base regularly rounded ; poste- 

 rior and anterior margins rounded from below the ears to the 

 base. Anterior ear triangular, flattened so as to be very dis- 

 tinct from the umbonal slope; posterior ear somewhat larger 

 than the other, compressed, triangular, the hinge side being 

 longer than either of the others, rather acutely angular at the 

 extremity. Beak compressed, a little nearer the anterior than 

 the posterior extremity of the hinge ; umbonal slopes diverging 

 from the beak at an angle of about 78 ° ; anterior one subangu- 

 lar. Surface ornamented by about fifteen regular, very promi- 

 nent, slender, and obscurely crenulated, concentric costae, which 

 are separated by spaces from four to six times their own 

 breadth, excepting on and near the ears; spaces between the 

 costae occupied by numerous fine, regular, closely arranged 

 concentric striae, which are crossed by faint indications of radi- 

 ating ribs. Diameter, from the pallial border to the hinge, 

 0.60 inch; breadth, from the anterior to the posterior margin, 

 0.62 inch; convexity, 0.12 inch. Right valve unknown. 



This exceedingly beautiful species seems to have varied a little in the details 

 of its surface markings at different ages. In young shells, the regular concen- 

 tric costae seem to be nearly or quite smooth, but as the shell advanced in its 

 growth, they gradually assumed an irregularly crenulated outline, and become 

 themselves ornamented with extremely fine longitudinal striae, not visible with- 

 out the aid of a lens. These striae are very much finer and less distinct than 

 those occupying the spaces between the costae. The faint indications of radi- 

 ating costas, seen crossing the concentric striae in the depressions between the 

 concentric ribs, are also wanting oh young shells. 



We know of no other species, from either American or foreign localities, 

 nearly allied to this. It is probably the most elaborately ornamented species 

 of the genus known, and may present some corresponding differences in the 

 hinge and interior, that would warrant its separation as the type of a distinct 

 genus. 



Locality and position: La Salle, Illinois ; Upper Coal Measures, near horizon 

 of coal No. 11. 



