_20993. Bering Island._—Adult male (?). (Type of _Z. grebnitzkii_.) These teeth are almond-shaped and very symmetrical. They are thickest near the base and taper gradually to the tip, which is quite acute. They are somewhat compressed and hence elliptical in section, the vertical diameter being greater than the transverse diameter. One side (probably the inner) is flattened. They are slightly curved upward toward the apex, which is a little worn and fractured. The root is very short and conical. It is nearly closed, but a very small canal extends upward for about 10 mm. The surface of the tooth is quite smooth, but dull in the lower half. The line of demarcation between cement and dentine is not evident. (Pl. 38, figs. 8, 9; pl. 23, fig. 1.) _21248. Bering Island._—Adult male (?). In this specimen the teeth are still in the natural position in the jaw. They are held in place by ligaments and protrude far beyond the alveoli, only about one-ninth of their length being below the superior border. They incline forward at an angle of about 45° with the longitudinal axis of the jaw and diverge slightly at the tips. The teeth themselves have the same general form as those of the preceding specimen, but are larger. The inner surface is flattened and the outer strongly convex. The tips are quite pointed, but show some indications of wear. The roots can not be seen distinctly, but appear to be closed. (Pl. 22, fig. 4.) _49599. Newport, Rhode Island._—Adult male. These teeth are longer than those of the preceding specimen, and while they resemble the latter in general form, taper much more gradually to the tip. The root, or portion below the point of maximum girth, is much shorter than that above, and rugose, with several deep furrows. A very small circular opening at the base of the root marks the orifice of the nerve. The upper half of the teeth is smooth, and the tips slightly worn and fractured. The small elliptical worn area is situated on the convex side of the tooth, which appears to be the outer side. As the alveoli of the jaw are, however, filled with a network of bone, the teeth can not be inserted in them. They were detached when received. (Pl. 38, figs. 10, 11; pl. 22, fig. 2; pl. 23, figs. 2, 3.) Besides the difference in the size and form of the teeth in the two sexes, it is probable, as will be seen by consulting the foregoing data, that in the female the apex of the teeth does not extend more than a very small distance above the alveoli even in mature individuals, and probably often not more than a few millimeters; while in adult males the teeth are almost entirely protruded from the alveoli, which are filled with a coarse bony network. These differences are carried out in all the American specimens, and also characterized the New Zealand specimens, as may be learned from the accounts of Haast and Hector.