268 ILLINOIS ACADEMY OF SCIENCE 
concave from which depression the curvature is rather steep 
to the lateral margins and from the umbonal region to the car- 
dinal area, the slope in less abrupt over the cardino-lateral 
margins, beak moderately prominent, strongly incurved. 
The surface of both valves on each side of the fold and sinus 
is marked by 5 to 7 low, bifurcating plications which are pro- 
gressively weaker toward the cardinal extremities; the bot- 
tom and sides of the mesial sinus are occupied by 3 to 5 sim- 
ilar plications, and a prominent furrow extends along the 
medial portion of the fold. Besides these stronger radiating 
markings, the surface of the plications and intervening depres- 
sions over the entire valves is ornamented by fine radiating 
lines; a few concentric lines of growth are often prominent 
near the anterior and lateral margins of the valves. The di- 
mensions of the type specimens are: length 32 mm., width 
39 mm., thickness 18 mm. 
This species may be easily distinguished by its large size, 
the well defined fold and sinus extending nearly to the beaks, 
and the fine radiating striae that cover the coarser radiating 
markings. Shells of this species occur in the Fernvale lime- 
stone near Thebes, Illinois, and at Cape Girardeau, Missouri. 
PELECYPODA 
Pterinea oblonga n. sp. 
Plate Vo iapsie: 
Description: In size and general outline this species more 
nearly resembles Pterinea demissa Conrad of the Richmond 
and Mayville beds than any other known species of the genus. 
It differs from Conrad’s species in the longer and less obtuse 
anterior auriculation, and in the much shorter and less acute 
extension at the posterior extremity of the hinge line. Our 
shell is semielliptical in outline, exclusive of the auriculations ; 
the right valve is oblique, longer than high, the greatest length 
along the hinge line, moderately convex over the umbonal 
region, antero-lateral and postero-lateral regions flat or nearly 
so, ventral half of the valve flat or slightly concave. Beak sit- 
uated about one-fourth of the length of the cardinal margin 
from the anterior extremity. The anterior auriculation is 
slightly larger than the posterior one, depressed or flat, not 
