The tympanic bulla does not differ materially from that of _bidens_ in size or shape, as far as can be judged from the figures given in Van Beneden and Gervais’ Osteography (plate 26, figs. 4, 4_a_). The periotic is similar in size to the same bone in _bidens_, but the posterior end is more narrowly pointed and the anterior end is much lower, relatively. In _europæus_, as far as can be determined from the material at hand, the form and size of the earbone is similar to that of _stejnegeri_, but in the latter the anterior margin of the tympanic bulla is more nearly transverse and the posterior inferior groove is curved. (Pl. 35, fig. 2.) In the Annisquam skull, supposed to represent _densirostris_, although from a young individual, the earbone is very much larger, especially the periotic, which is also quite differently shaped. MANDIBLE. The mandible of _stejnegeri_ is much broken in the region of the angle on both sides, but otherwise complete. As compared with a mandible of an adult _bidens_, the most conspicuous differences are the shortness of the symphysis, the sharp upward bend of the inferior margin anteriorly, and the large size of the alveolus. The symphysis in the adult Oregon specimen of _stejnegeri_ is 140 mm. long, or scarcely more than in the young specimen of _europæus_ from New Jersey, and exactly the same as in the adult type-specimen of the latter species, as figured by Van Beneden and Gervais. The alveolus lies entirely behind the symphysis, its anterior end being 160 mm. from the anterior end of the jaw. It is 113 mm. long and 18 mm. wide. The mandible is 62 mm. high at its middle point. The coronoid process is more anteriorly situated than in _bidens_ and the portion of the posterior margin of the ramus which remains indicates that the angle was strongly directed backward. (Pl. 11, fig. 4; pl. 12, fig. 1.) TEETH. The teeth are remarkable for their size and form. They are somewhat more than twice as broad as teeth of adult males of _bidens_, as shown by the figures of Lankester[32] and Grieg,[33] and also a little longer. They are, in fact, probably broader than, or at least as broad as, the teeth of any other species of _Mesoplodon_, not excepting _layardi_. Sir William Turner remarks regarding a specimen of _layardi_ examined by him that “the breadth of the tooth, where it emerged from the alveolus, was 3½ inches.”[34] He does not state, however, whether the measurement was taken along the top of the alveolus, at an angle with the transverse axis of the tooth, or along the transverse axis itself. At all events, the teeth figured by Owen and others are much less than 3½ inches broad. The teeth of adult _europæus_ are only 2 inches broad, and of _bidens_, as already stated, 1½ inches broad.