332 
Aug. F. Foerste 
strongly indicated. The mesial depression anterior to the umbonal 
ridge is slight. There practically is no mesial sinus along the ventral 
margin, the latter being straight along the base of the mesial de- 
pression. 
39. Modiolopsis brevantica, n. sp. 
{Plate V, Figs. 1 A, B) 
Species apparently belonging to the same group as Modiolopsis 
concentrica, but much more convex. Anterior margin sloping ob- 
liquely downward, and projecting only 3 millimeters anterior to 
the beak. Posterior part of the cardinal margin only slightly convex 
for a distance of 15 millimeters from the anterior part of the beak, 
and then deflected downward at an angle of about 140 degrees. 
The umbonal ridge has about the same direction, 140 degrees with 
the horizontal. The shell is swollen out along the umbonal ridge, 
the greatest distance from valve to valve equalling 12 millimeters, at 
a distance of 12 millimeters from the beak. The shell is vertically 
compressed along the base, but it is evident from the concentric 
lines ornamenting the surface that it was long and slender. The 
base has about the same direction as the umbonal ridge, and there- 
fore also forms an angle of about 140 'degrees with the horizontal. 
The lowest part of the base reaches about 21 millimeters below the 
horizontal continuation of the hinge-line. The greatest width of 
the shell between the basal and posterior outlines equals about 16 
millimeters. Both anteriorly and posteriorly the shell is quite 
evenly rounded. The mesial sinus is shallow and not strongly 
defined. The umbonal ridge is fairly angular anteriorly and is much 
more strongly defined than in Modiolopsis concentrica. The beaks 
are small and approach each other closely. The preumbonal slopes are 
flattened. Viewed from above, the shell has an elliptical-lanceolate 
outline. The surface is quite strongly ridged concentrically, 7 ridges 
occupying a length of 5 millimeters along the umbonal ridge. 
Found in the Waynesville member of the Richmond at Clay 
•cliff, 3 miles north of Wekwemikongsing, on the eastern shore of 
Manitoulin island, Canada. 
40. Pholadomorpha pholadiformis, Hall 
1851. Modiolopsis pholadiformis Hall, Geol. Lake Superior Land. Dist., Foster and 
Whitney Rep., p. 213, Plate 30, Figs. 1 a-c; Plate 31, Fig. 1 
Pholadomorpha pholadiformis is characterized so strongly by 
its peculiar surface ornamentation that it seems almost incredible 
