2S The American Geologist. May, 182 
It can, however, be separated readily from both when the interior 
is shown, by the strong mesial septum of the pedicle (ventral) 
valve. This originates between the diductor sears and continues 
to increase in strength to near the anterior margin where it often 
coalesces with one or two of the vascular ridges. In S. trenton- 
cnsis the thickening of the interior near the anterior margin of the 
pedicle valve is obsolete, or entirely undeveloped—whieh is 
another distinguishing feature. The cardinal processes in beth 
these species ure also more elevated, while the rostral thickening 
upon which it rests is less strongly developed than in S. rugosa. 
The septum of the pedicle (ventral) ‘valve in S. septate, will also 
distinguish it from S. ¢neurvata, in addition to its smaller size, 
and comparatively greater width than length. 
Plate xxx1, figs. 1-3. 
Formation and locality. Common in the upper third of the Trenton 
shales at St. Paul and Minneapolis, and at Pleasant Grove, Olmsted Co., 
Minn. 
Strophomena planodorsata. 
Large, semi-circular or sub-quadrate in outline, comeayo-con- 
vex, wider than long, greatest width along the hinge-line or 
immediately in front of it. Surface with fine radiating strizw every 
other or every third one somewhat stronger than those intermedi- 
ate, crossed by exceedingly delicate and closely crowded concen- 
tric lines, and towards the anterior margin by a few larger sub- 
imbricating lines of growth. 
The size, large flattened area of the dorsal valve, and the sub- 
quadrate muscular area distinguish this species from all others of 
the S. rugosa type. 
Plate xxx1, figs. 8-10. 
Formation and locality. Hudson River group, Spring Valley; Tron 
tidge, Wis.: Wilmington, Il. 
Strophomena scofieldi. 
Shell small, semi-circular in outline, biconvex, with a more or 
less prominent fold and sinus toward the anterior margin; hinge- 
line a little shorter than the greatest width; area of pedicle (ven- 
tral) valve forming an angle of about 140° with the plane of the 
lateral margin, centrally occupied by a convex perforated delti- 
dium; that of the brachial (dorsal) valve fitting closely against 
the other valve. Surface marked by numerous crowded, 
