In the _MYSTACOCETI_ the skull is always quite bilaterally symmetrical, and is not so much modified from the ordinary mammalian type as in the Odontoceti. The parietals are not, as in the Odontoceti, separated by a wide interparietal, but meet; they are, however, hidden under the very large supra-occipital. The nasals are developed to a certain extent, and the nares, though placed very far back and near the top of the head, terminate forwardly-directed narial passages. Turbinal bones are also developed to some extent; this fact, and the occurrence of a definite though small olfactory fossa constituting important distinctions from the Odontoceti. The maxillae are large, but do not extend back to cover the frontals as in the Odontoceti. The zygomatic process of the squamosal is very large. The mandibular rami are not compressed, but are rounded and arched outwards, and never meet in a long symphysis. _ODONTOCETI._ The skull departs widely from the ordinary mammalian type. The following description will apply to any of the following genera of the Delphinidae, _Phocaena_, _Globicephalus_, _Lagenorhynchus_, _Delphinus_, _Tursiops_, _Prodelphinus_, _Sotalia_. The upper surface of the skull is more or less asymmetrical. The cerebral cavity is high, short and broad; and formed mainly by the cerebral fossa, the olfactory fossa being entirely absent. The supra-occipital (fig. 93, 3) is very large, and forms much of the posterior part of the roof of the skull. It has the interparietal (fig. 93, 7) fused with it, and completely separates the two parietals. The frontal (fig. 93, 10) is large and laterally expanded, forming the roof of the orbit, but is almost completely covered by an extension of the maxillae. The zygomatic arch is very slender, and is mainly formed by a rod-like process from the jugal (fig. 93, 15), the zygomatic process of the squamosal being short and stout. The nasal passages are peculiarly modified, instead of passing horizontally forwards above the roof of the mouth, they pass upwards and even somewhat backwards towards the top of the skull (fig. 93, 23). They are bounded laterally by two processes from the premaxillae, the left of which is shorter than the right. The nasal cavities are narrow and without turbinals and the nasals (fig. 93, 19) are almost as much reduced as in Sirenia. [Illustration FIG. 93. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF THE SKULL OF A YOUNG CA'ING WHALE (_Globicephalus melas_) × 1/6. (Brit. Mus.) 1. basi-occipital. 2. exoccipital. 3. supra-occipital. 4. basisphenoid. 5. alisphenoid. 6. parietal. 7. interparietal. 8. presphenoid. 9. orbitosphenoid. 10. frontal. 11. mesethmoid. 12. tympanic. 13. periotic. 14. squamosal. 15. jugal. 16. vomer. 17. palatine. 18. pterygoid. 19. nasal. 20. maxillae. 21. premaxillae. 22. mandible. 23. anterior nares.]