284 
Aug. F. Foerste 
belongs to the group of shells having a comparatively straight 
hinge-line posterior to the beak, and a moderately convex or 
nearly straight, instead of concave basal margin; there is no 
mesial sulcus; and the anterior margin of the shell extends con- 
spicuously in front of the beaks. 
20. Modiolopsis postplicata, sp. nov. 
{Plate I, Fig. 4) 
Hinge-line arcuate posterior to the beak, gently declining to- 
ward the posterior extremity and gradually rounding into the 
oblique posterior margin; umbonal ridge broad and low, rounded 
rather than angiilate; mesial sulcus shallow, forming an undefined 
depression across the valves from the beak toward a point slightly 
anterior to the middle of the basal margin, where the latter is 
slightly concave; cardinal or post-umbonal slope marked by regu- 
lar, even, concentric plications becoming obsolete toward the 
crest of the umbonal ridge; these plications mark successive 
growth stages of the posterior margin of the shell ; there are about 
5 plications in a length of 5 mm., increasing to 6 in the same 
length posteriorly; within 10 mm. from the posterior margin, fine 
concentric striae are seen in addition to the plications. Concen- 
tric striations are present also on the areas below the umbonal 
ridge; these striations are faint posteriorly, but become sharply 
defined anteriorly, where they also are more crowded. The 
valves probably were very thin, since not only the position of the 
anterior muscular scar, but also that of the anterior part of the 
pallial line, for a distance of 10 mm., is distinctly indicated. 
Both the cardinal and the basal margins are curved, the curvature 
of these margins being approximately equal, the margins being 
subparallel. 
Length 58 mm., height 18 mm., height at beak 16 mm., exten- 
sion of shell anterior to the beak about 10 mm., convexity of the 
single valve about 4 mm. 
Locality. Type, associated in same rock fragment with Lopho- 
spira heatrice and Pterinea demissa; collected on the Riviere des 
Hurons, near St. Jean Baptiste, October 1872, by Thomas Curry; 
preserved in the Paleontological collections of the Geological 
Survey of Canada, in the Victoria Memorial Museum, Ottawa, 
Canada. No. 8424. 
