the limits of the preserved specimen. It probably bifurcates farther out from the calyx. The cross-section of the arms is circular and the shape of the arm plates resembles a truncated cone with the base upward. It is not quite clear whether the number of arms is five or six because the branching in one or two cases begins so near the calyx. Dorsal canal extending throughout the arms.
Horizon and locality.—Onondaga limestone, north shore of Lake Erie, three and one-half miles east of Port Dover, Ontario.
Poterioceras canadensis n.sp.
Plate I, Figs. 2-5
Shell small, tapering both ways from the base of the chamber of habitation or last air chamber, and extending to a rather blunt point at the apex. Ventral side strongly curved, dorsal side nearly straight but curving slightly upward near the apex. Transverse section subcircular.
Chamber of habitation relatively large, being about two-fifths of the length of the shell, and more or less pear-shaped.
Air chambers regular, increasing slightly in thickness from the apex to the chamber of habitation. Septa smooth, thin, and concavity rather slight. Suture straight and horizontal.
Siphuncle small and marginal on the ventral side.
Aperture subtriangular. Hyponomic sinus well developed.
Surface of shell nearly smooth, marked only by fine lines of growth.
Horizon and locality.—Onondaga limestone. Hamilton's Quarry, Gorrie, Ontario.
Rhipidomella intermedia n.sp.
Plate II, Figs. 1 and 2
Shell subcircular and more or less lenticular in transverse section. Hinge line equal to slightly more than half the width of the shell. Both valves are convex. The pedicle valve with a flattened area along the median line just in front of the middle and extending to the front margin, where it becomes a broad indistinct sinus. Over the corresponding surface of the brachial valve there is a broad convexity.
