54 
AUG. F. FOERSTE 
120 degrees with the cardinal margin. Along this part of its 
outline the posterior margin ^usually is only moderately convex, 
there being a distinct tendency toward straightening. The an- 
terior part of the valve extends from 3.5 to 4 mm. in front of the 
beak. It is fairly evenly convex, its maximum convexity appear- 
ing slightly below mid-height of the valve, on account of the 
slight inward curvature of this outline on approaching the um- 
bonal part of the valve. The umbonal part is relatively broad, 
and only slightly elevated above the cardinal margin. The um- 
bonal ridge is sigmoidal in direction, starting near the beak at a 
small angle with the cardinal margin, this angle increasing to 20 
and 25 degrees near mid-length of the valve, and then decreas- 
ing again posteriorly as far as the posterior angle of the valve. 
The post-umbonal slopes of the valves are distinctly concave, 
but the umbonal ridge tends to be rounded, rather than angular. 
In most specimens the middle part of the valves, beneath and 
anterior to the umbonal ridge, is gently convex antero-poste- 
riorly, with a tendency toward flattening where the mesial sulcus 
might appear, this sulcus usually being absent, though faintly 
indicated occasionally. 
Remarks. — These specimens from the Whirlpool sandstone 
at Credit Forks, Ontario, differ from typical Modiolopsis ortho- 
nota from the Grimsby sandstone at Lockport, and Medina, New 
York, in the greater angulation of the posterior side of the um- 
bonal ridge and in the more distinctly concave curvature of the 
post-umbonal slope, between the umbonal ridge and the hinge- 
area. 
Owing to its more angular umbonal ridge, the specimen 
represented by figure 1 on plate XIII may be designated as 
Modiolopsis orthonota perumbonata Var. nov. 
In the type specimens of Modiolopsis orthonota that part of 
the shell which corresponds to the angular part of the umbonal 
ridge of the Credit Forks specimens is more evenly rounded 
while the distinctly concave part is narrower, its lower margin 
deviating only 2.5 mm. from the hinge-line at the posterior end 
of the shell. Immediately beneath the beak of the right valve 
the hinge-area appears to have been slightly elevated in a more or 
