_Extinct Genera._—The extinct genus _Cetotherium_ of the European Pliocene may be taken to include a number of fossil Whalebone Whales allied to the Balænopterine group, several of which have been described under other names, such as _Plesiocetus_, _Heterocetus_, and _Amphicetus_. They are readily characterised by the form of the tympanic bone, which is much narrower in front than behind, the roughened inferior surface being in the shape of an isosceles triangle, and the notch for the Eustachian canal being smaller, and descending nearer to the inferior border of the inner wall than in _Balænoptera_. The skull is longer than the latter, with a greater interval between the occiput and the frontal, and with longer and more flattened nasals. The relative thickness of the cervical vertebræ is also greater. In the typical forms (_e.g._ _C. brialmonti_ and _C. dubium_) the mandibular condyle is simple; but in _C. (Heterocetus) brevifrons_ it is furnished with a projecting posterior talon, as in the Sperm Whale. _Herpetocetus_ is known by a comparatively small species from the Belgian and English Crags, characterised by the extreme inflation of the egg-shaped tympanic bone, which approximates to that of _Megaptera_, but has the greater part of the cavity filled by bone. There is a talon to the condyle of the mandible. _Palæocetus_, as already mentioned (p. 232), is founded upon the ankylosed cervical vertebræ of a small Whale originally considered as having been derived from the Kimeridge Clay, but which doubtless came from the Suffolk Crag; if it belongs to the _Balænidæ_ it indicates a Right Whale. _Suborder_ ARCHÆOCETI. _Family_ ZEUGLODONTIDÆ. This group is formed to include certain extinct Cetacean-like animals at present only known by more or less fragmentary portions of their skeleton and teeth, and whose position and affinities are, therefore, still subject to doubt.[138]