In both the living genera the dentition is monophyodont. In _Manatus_ the dentition is _i_ 2/2 _pm_ and _m_ 11/11. The incisors are vestigial, and disappear before maturity. The grinding teeth have square enamelled crowns marked by transverse tuberculated ridges. They are not all present in the jaw at the same time. In _Halicore_ the upper jaw bears a pair of straight tusklike incisors; in the male these have persistent pulps and project out of the mouth; in the female they soon cease to grow and are never cut. They are separated by a long diastema from the grinding teeth which have tuberculated crowns and are 5/5 or 6/6 in number, but are not all in place at once. Several other pairs of slender teeth occur in the young animal, but are absorbed or fall out before maturity. In _Rhytina_ teeth are altogether absent. CETACEA. _ARCHAEOCETI._ _Zeuglodon_ has the following dentition, _i_ 3/3 _c_ 1/1 _pm_ and _m_ 5/5, total 36. The incisors and canines are simple and conical; the cheek teeth are compressed and have serrated cutting edges like those in some seals. In the _MYSTACOCETI_, or whalebone whales, calcified tooth germs probably belonging to the milk dentition are present in the embryo, but they are never functional, and are altogether absent in the adult. The anterior of these germs are simple, the posterior ones are originally complex, but subsequently split up into simple teeth like those of the anterior part of the jaw. Hence according to Kükenthal, who described these structures, the Cetacean dentition was originally heterodont. In the living _ODONTOCETI_ the dentition is homodont and monophyodont. In some cases traces occur of a replacing dentition which never comes to maturity, and renders it probable that the functional teeth of the Odontoceti are really homologous with the milk teeth of other mammals. Some of the dolphins afford the apparently simplest type of mammalian dentition known. The teeth are all simple, conical, slightly recurved structures, with simple tapering roots and without enamel. The dentition is typically _piscivorous_, being adapted for seizing active slippery animals such as fish. The prey is then swallowed entire without mastication. Sometimes the teeth are excessively numerous, reaching two hundred or more (fifty to sixty on each side of each jaw) in _Pontoporia_. This multiplication of teeth is regarded by Kükenthal as due to the division into three parts of numbers of trilobed teeth similar to those of some seals.