In the Tapiridae the grinding teeth are brachydont and the lower ones are typically bilophodont. The last two upper molars have the transverse ridges united by an outer longitudinal ridge. The dentition is i 3/3 c 1/1 pm 4/3 m 3/3, total 42.
In some of the extinct Perissodactyles such as Lophiodon[160], the dentition is brachydont and bilophodont, the grinding teeth in general resembling the posterior upper molars of the Tapir. The same type of brachydont tooth is seen in Palaeotherium but the transverse ridges are crescentic instead of straight, and are separated from one another by shallow valleys without cement. Some of the Palaeotheridae have the regular series of forty-four teeth.
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.
Fig. 82. Palatal aspect of the cranium and mandible of a Donkey (Equus asinus) × 1/5. (Camb. Mus.)
| 1. supra-occipital. | 6. glenoid surface. |
| 2. occipital condyle. | 7. vomer. |
| 3. basi-occipital. | i 1, i 3. first and third incisors. |
| 4. vacuity representing the | c. canine. |
| confluent foramen lacerum | pm 1, pm 2. first and second |
| posterius and foramen | premolars. |
| lacerum medium. | m 1. first molar. |
| 5. auditory bulla. |