PAPERS ON GEOLOGY 267 
Dorsal valve about equal in convexity to the ventral, the 
posterior margin more obtuse and the posterior lateral margins 
more convex than those of the ventral valve; a rather promi- 
nent internal median ridge is indicated by a depression in the 
surface of the impression of the interior extending from near 
the apex anteriorly to near the mid-length of the valve. Sur- 
face marked by concentric lines similar in character to those on 
the ventral valve. 
The dimensions of the type specimen are: length 14 mm., 
width 8 mm., thickness about 2 mm. Shells of this species are 
sparingly present throughout the Thebes sandstone in Illinois. 
In size and general appearance this species resembles Lin- 
gula covingtonensis from the Trenton limestone, but may be 
distinguished from that form by the less acuminate posterior 
portion, the narrower width of the shell in proportion to its 
length, and the more flattened character of the concentric 
ridges. 
Hebertella lineolata n. sp 
Plate I, Figs, 1 and 2. 
Description: Shell subquadrate in outline, wider than long, 
the hinge line equaling, or a little shorter than the greatest 
width, the cardinal extremities obtusely angular to subrec- 
tangular. 
Ventral valve moderately convex, the greatest depth near the 
middle on each side of the sinus, the surface sloping rather 
steeply from the beak and borders of the sinus to the front and 
lateral margins, but becoming gently convex towards the car- 
dinal extremities ; beak moderately prominent, arched ; cardinal 
area of moderate width, gently concave, mesial sinus beginning 
in the umbonal region, and becoming wider and deeper to the 
front where it is somewhat produced, the bottom of the sinus 
flat or gently concave. 
Dorsal valve strongly convex, the greatest depth in front 
of the umbonal region, the medial portion elevated into a flat- 
tened mesial fold which begins near the beak and becomes 
broader and increasingly prominent to the front where the 
valve is emarginate; bordering each side of the fold in the 
anterior portion of the shell the surface is flattened or slightly 
