The genus _Iniopsis_ of Mr. Lydekker,[264] with one species, _I. caucasica_, comes from rocks which seem to be of that age. The back part of the skull of this animal, the only part of the skull known, has the same squarish excavation of the maxillaries that characterises _Inia_ and _Pontoporia_. Its lower jaw was slender and possessed numerous teeth. The long snout and jaws of Platanistids, especially exaggerated in _Pontoporia_ among living forms, are constantly found in these Tertiary Platanistids. _Eurhinodelphis_ had a beak three and a half times the length of the cranium, whereas in _Pontoporia_ the proportions are as 2:1. The teeth too were very numerous. The genus _Argyrocetus_, from Patagonian Tertiary strata, was an animal about as large as the existing Dolphin. It had the {384} slender rostrum and numerous teeth of the Platanistids and the squared excavations of the maxillaries. _Argyrocetus patagonicus_ possessed also archaic characters, suggesting earlier affinities still. The two condyles of the skull instead of being closely adpressed to the skull stood out in a way more like that met with in terrestrial mammals. The nasal bones instead of being abbreviated rudiments are well developed as in the archaic Zeuglodonts. The cervical vertebrae of this Whale are all perfectly free from each other and individually long. The skull is on the whole bilaterally symmetrical; this again is a feature more pronounced among the Platanistidae than among other Odontocetes. Accompanying these generalised Cetacean characters are some which show that the animal was too specialised to be the direct ancestor of any existing forms. The end of the mandible was upturned and without teeth, its form being quite unique among Cetacea. Other allied forms, such as _Zarrhachis_ and _Priscodelphinus_, showed the same length of the cervical vertebrae. A very distinct family of extinct Whales is that of the SQUALODONTIDAE. They to some extent bridge over the gap between the existing Odontoceti and the Eocene Archaeoceti (Zeuglodonts). The skull of these Whales was on the whole Dolphin-like. But they possessed teeth which were distinctly specialised into incisors, canines, and molars. The molars have a coarsely-serrated cutting edge as in the Zeuglodonts, and are also to some extent two-rooted. But they are more numerous, and so far approximate to the conditions which characterise the more typical modern Odontocetes. _Squalodon_ was a long-beaked form, and _Prosqualodon_ had a skull whose proportions are nearer those of _Kogia_. SUB-ORDER 3. ARCHAEOCETI. This division of the Whale tribe embraces but a single family, ZEUGLODONTIDAE, of which but a single genus, _Zeuglodon_, can with certainty be discriminated.