In _stejnegeri_ (Pl. 12, figs. 1-3) the portion of the tooth above the alveolus is inclined slightly inward and backward, but the pointed tip curves outward so as to be vertical. When extracted from the alveolus, the whole tooth is found to be concave internally and convex externally. The posterior margin is convex and the anterior sinuous, a slight convexity occurring on the portion which projects above the alveolus. In this place the outer coating of cement is broken through, showing the underlying dentine or osteo-dentine, which is somewhat corroded or absorbed. This is particularly noticeable on the left tooth. The upper margin of the tooth is transverse, or nearly at right angles with the anterior and posterior margins. The posterior angle is rounded and the anterior raised into an acute point by the projection of the dentine as a distinct, sharp cusp. The inferior end of the tooth is cut off obliquely and the margin is broken by numerous prominent rugosities. The surface of all that part of the tooth which is contained in the alveolus and covered by the gum above it is rugose, while the part above the gum is quite smooth and highly polished. The right tooth has the following dimensions (in straight lines): Length of anterior border, 150 mm.; length of posterior border, 107; length of superior border, 54; length of inferior border, 86; average length of exposed dentine tip, 10; greatest breadth of tooth, antero-posteriorly, 81; greatest breadth of tooth, transversely, 15; distance from center of base of exposed portion, when in position in the alveolus, to tip of dentine projection, 82; distance from center of base of portion above the gum to tip of dentine projection, 70; distance from center of base of portion above the gum to center of inferior margin, 76. The dimensions of the skulls are as follows, those of the type-specimen having been revised and corrected: _Dimensions of two skulls of M. stejnegeri._ Column headings: A: 143132 U.S.N.M. Yaquina Bay, Oregon, adult. B: 21112 U.S.N.M. Bering Id. Type (1715), young.