578 PALEONTOLOGY OF ILLINOIS. 



boniferous species with which we are acquainted, though of course pre- 

 senting well marked specific differences. All of the twenty-odd speci- 

 mens we have seen are left valves, from which fact we may infer that 

 the right valve was more fragile, and consequently rarely preserved. 

 It is also probable that it was less convex, and more faintly marked 

 than the other, as is usual in shells of this kind. As we know nothing 

 of the hinge and muscular impression of this shell, we of course cannot 

 determine whether it is a Pterinea or a true Plcria (AvicitlaJ. If the 

 latter, and KLEIN'S older pre-Liniiaean names are to be adopted, its 

 uanie will become Avicula Morganemis. 



Locality and position Below the middle of the Coal Measures; Mor- 

 gan county, Illinois. 



AVICULA LONG A, Geinitz sp. 



PL 26, Fig. 1. 

 Gemllia longa, GEIMTZ, 1866. Carb. and Dyasin Nebraska, p. 32; taf. 2, fig. 15. 



Locality and position Danville, Illinois 5 roof shales of Coal No. 7. 



Genus PLACUM)PSIS. 

 PLACUNOPSIS CAKBONARIA, M. and W. 



PI. 27, Fig. 2. 

 Placunopsis earbonaria, MEEK and WOUTHEN, 1866. Proceed. Chicago Acad. Sci., Vol. I, p. 13. 



SHELL orbicular, compressed sub-hemispherical, ex- 

 tremely thin. Upper or right valve irregularly convex, 

 sometimes rather gibbous; hinge margin straight, gener- 

 ally equaling about one-third to one-half the diameter of the 

 valves, and usually showing a slight disposition to develope 

 small, obtuse ears at the extremities; beak very small, 

 sometimes compressed and nearly obsolete, marginal, but 

 not projecting distinctly beyond the cardinal border, located 

 at the middle of the hinge, and showing a slight forward 

 curvature. Under valve flat, or conforming to the inequal- 

 ities of the surface to which it was attached; beak marginal, 

 and very nearly obsolete. Surface of both valves marked 

 A\ ith irregular, undulating or interrupted radiating lines, 



