PAPERS ON GEOLOGY 269 
sharply defined from the body of the shell, the anterior margin 
meets the hinge line at an angle of about 50 degrees; is slightly 
sinuate below the auriculation below which it curves with 
gentle convexity to the broadly rounded ventral margin; pos- 
terior auriculation nearly flat, not sharply defined, the posterior 
margin forming an angle of about 45 degrees with the hinge 
line, below the auriculation the posterior border is nearly 
straight to near the lower part, where it blends with a broad 
curve into the ventral margin. Entire surface marked by 
numerous rather fine concentric lines of growth which are 
most conspicuous over the auriculations. A few indistinct 
radiating lines may also be present. 
The dimensions of the type are: length along the hinge line 
45 mm., greatest length below the auriculations, 37.5 mm., 
height 37.5 mm., convexity of right valve, 3 mm. 
Shells of this species are rather rare in the Orchard Creek 
shale south of Thebes, Illinois. 
GASTROPODA 
Strophostylus ornatus n. sp. 
Plate I, Figs. 4 and 5 
1913 cf. Cyclomema cancellata Hall, Ill., State Geol. Sur- 
vey, Extract Bulletin No. 23, p. 55, pl. 4, fig. 4. 
Description: Shells resembling those of S. cancellatus Hall, 
to which species they were referred in a former report. How- 
ever, the species here described may be distinguished from S, 
cancellatus by the more shallow suture, and the marked dif- 
ference in surface ornamentation; in our species the revolving 
lines are much more numerous, and continue in full strength up 
to the suture, and into the umbilicus. The shell is turbinate, 
height and width about equal, volutions about 4; apex small, 
upper volutions increasing rather rapidly in size to the body 
whorl which enlarges more rapidly forming from two-thirds 
to three-fourths of the height of the shell; the whorls are sub- 
ovate in cross section, umbilicus shallow, inner lip slightly re- 
flexed, outer lip not expanded, sutures not deeply impressed. 
The height of different shells varies from 19 to 22 mm. and 
the width from 16.5 to 22 mm. The entire surface is orna- 
