_Ziphius._[146]—A single conical tooth of moderate size on each side of the mandible close to the anterior extremity, and directed forwards and upwards. Skull with the premaxillæ immediately in front of, and at the sides of the nares expanded, hollowed, and with elevated lateral margins, the posterior ends rising to the vertex and curving forwards, the right being considerably more developed than the left; the conjoint nasals forming a strongly pronounced symmetrical eminence at the top of the cranium, projecting forwards over the nares, flat above, most prominent and rounded in the middle line in front, and separated by a notch on each side from the premaxillæ. Anteorbital notch not distinct. Rostrum (seen from above) triangular, gradually tapering from the base to the apex; upper and outer edges of maxillæ at base of rostrum raised into low roughened tuberosities. Mesethmoid cartilage densely ossified in adult age, and coalescing with the surrounding bones of the rostrum. Vertebræ: C 7, D 10, L 10, C 22; total 49. The three anterior cervical vertebræ united, the rest free. The type of this genus is _Z. cavirostris_ of Cuvier, founded upon an imperfect skull picked up in 1804 on the Mediterranean coast of France, and described and figured in the _Ossemens Fossiles_ under the impression that it was that of an extinct species. Many other individuals have, however, been subsequently met with in various parts of the world, from the Shetland Islands to New Zealand, all referable to the same genus, if not to the same species; although, as is usual in such cases, they have mostly been described under different names, the so-called genera _Petrorhynchus_ and _Epiodon_ being probably referable to the type species. It is quite probable that some of the Physeteroid teeth from the Crag deposits mentioned on p. 251 may be referable to _Ziphius_. [Illustration: FIG. 85.—_Mesoplodon bidens._ From Reinhardt.] _Mesoplodon._[147]—A much compressed and pointed tooth in each ramus of the mandible, variously situated, but generally at some distance behind the apex (Fig. 86); its point directed upwards, and often somewhat backwards, occasionally developed to a great size. Skull with the region around the nares as in _Hyperoödon_, except that the nasals are narrow and more sunk between the upper ends of the premaxillæ; like those of _Hyperoödon_, they are concave in the middle line in front and above. No maxillary tuberosities. Anteorbital notch not very distinct. Rostrum long, narrow, and solid throughout. Mesethmoid in adult age ossified in its entire length, coalescing with the surrounding bones, and showing as a narrow band on the upper surface of the rostrum. Vertebræ: C 7, D 10, L 10 or 11, C 19 or 20; total 46 to 48. Two or three anterior cervicals united, the rest usually free. [Illustration: FIG. 86.—Left lateral view of skull of _Mesoplodon densirostris_.]