A complete series of forms is known showing how from the simple brachydont teeth of the Palaeotheridae, were derived the complicated hypsodont teeth of the Equidae. The increase in depth of the tooth was accompanied by increase in the depth and complexity of the enamel infoldings, and of the cement filling them. Both upper and lower grinding teeth of the Equidae are much complicated by enamel infoldings, but their derivation from the bilophodont type can still be recognised. The diastema in front of the premolars is longer in the living Equidae than in their extinct allies. In the adult horse the dental formula is _i_ 3/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3, total 40, with often a vestigial first upper premolar (fig. 82, _pm_ 1). The last molar is not more complex than the others, and in the female the canine is quite vestigial. The incisors are large and adapted for cutting and have the enamel curiously folded in forming a deep pit. The milk dentition is _di_ 3/3 _dc_ 0/0 _dpm_ 3/3, total 24. The last milk premolar is not more complex than the premolar that succeeds it. The horse affords an excellent instance of a typically _herbivorous_ type of dentition, the cutting incisors, reduced canines and series of large square flat-crowned grinding teeth being most characteristic. In _Rhinoceros_ the grinding teeth are much like those of _Lophiodon_, having an outer longitudinal ridge from which two crescentic transverse ridges diverge. The upper premolars are as complex as the molars, and there are no canines; in some species incisors also are absent. The dental formula is _i_ (0--2)/(0--1) _c_ 0/(0--1) _pm_ 4/4 _m_ 3/3. [Illustration FIG. 82. PALATAL ASPECT OF THE CRANIUM AND MANDIBLE OF A DONKEY (_Equus asinus_) × 1/5. (Camb. Mus.) 1. supra-occipital. 2. occipital condyle. 3. basi-occipital. 4. vacuity representing the confluent foramen lacerum posterius and foramen lacerum medium. 5. auditory bulla. 6. glenoid surface. 7. vomer. _i_ 1, _i_ 3. first and third incisors. _c._ canine. _pm_ 1, _pm_ 2. first and second premolars. _m_ 1. first molar.] Among the Titanotheriidae _Palaeosyops_[161] has very brachydont teeth whose crowns have been described as _buno-selenodont_, the inner pair of columns being bunodont, the outer, selenodont. Similar grinding teeth occur in _Chalicotherium_. Some of the Titanotheriidae have the regular mammalian series of forty-four teeth. SUBUNGULATA. _TOXODONTIA._ _Nesodon_ has the regular dental formula; its grinding teeth are rooted and the upper ones resemble those of Rhinoceros. The second upper and third lower incisors form ever-growing tusks. There is a marked difference between the deciduous and permanent dentition. _Astrapotherium_ likewise has large rooted cheek teeth of a rhinocerotic type, and each jaw bears a pair of permanently growing tusks, those of the lower jaw being the canines. The dental formula is _i_ 1/3 _c_ 0/1 _pm_ 2/1 _m_ 3/3, total 28.