margin and partially bisected by a prominent rounded ridge impression. This latter was apparently longitudinally striated while the muscular scars themselves were concentrically marked.
Horizon and locality.—Amherstburg dolomite, Stony Island Dry Cut, Livingston Channel, Detroit River, near Trenton, Michigan.
Stropheodonta delicatula n.sp.
Plate II, Figs. 4 and 6
Shell semi-elliptical, somewhat wider than long. Cardinal extremities produced, hinge line usually greater than the greatest width of the shell.
The specimens found are chiefly brachial valves and of those only the external impression, with rarely a rather poor preservation of the shell. Fragments of the pedicle valve show that it was regularly convex and rather gibbous. The beak was small and slightly incurved and extending beyond the area as in Stropheodonta demissa. Brachial valve moderately to deeply concave, usually following rather closely the inside curvature of the pedicle valve, leaving scarcely a sixteenth of an inch as the probable thickness of the animal.
Area of pedicle valve arcuate and more or less triangular; area of the brachial valve apparently flat and uniform in width.
Surface of valves marked by strong radiating striae, rather distantly spaced, and between which are numerous fine striae. In the umbonal region the surface is marked by distinct ribs or costae, which seem to persist for half an inch or more from the beak and then are gradually lost. These radiating surface ornamentations are crossed by numerous growth lines which are occasionally aggregated into more conspicuous wrinkles. This species may be compared with Stropheodonta galatea.
Horizon and locality.—Amherstburg dolomite, Stony Island Dry Cut, in Livingston Channel, Detroit River, near Trenton, Michigan.
Nucleospira livingstonensis n.sp.
Plate II, Fig. 6
Shell nearly circular in outline, rather gibbous and rounded oval in transverse section. Width and length nearly equal.
