The chief palaeontological interests of the Pliocene deposits, as of the succeeding Post-Pliocene, centre round the Mammals of the period; and amongst the many forms of these we may restrict our attention to the orders of the Hoofed Quadrupeds (Ungulates), the Proboscideans, the Carnivora, and the Quadrumana.  Almost all the other Mammalian orders are more or less fully represented in Pliocene times, but none of them attains any special interest till we enter upon the Post-Pliocene.

Amongst the Odd-toed Ungulates, in addition to the remains of true Tapirs (Tapirus Arvernensis), we meet with the bones of several species of Rhinoceros, of which the Rhinoceros Etruscus and R.  Megarhinus (fig. 249) are the most important.  The former of these (fig. 249, A) derives its specific name from its abundance in the Pliocene deposits of the Val d’Arno, near Florence, and though principally Pliocene in its distribution, it survived into the earlier portion of the Post-Pliocene period. Rhinoceros Etruscus agreed with the existing African forms in having two horns placed one behind the other, the front one being the longest; but it was comparatively slight and slender in its build, whilst the nostrils were separated by an incomplete bony partition.  In the Rhinoceros megarhinus (fig. 249, B), on the other hand, no such partition exists between the nostrils, and the nasal bones are greatly developed in size.  It was a two-horned form, and is found associated with Elephas meridionalis and E.  Antiquus in the Pliocene deposits of the Val d’Arno, near Florence.  Like the preceding, it survived, in diminished numbers, into the earlier portion of the Post-Pliocene period.

[Illustration:  Fig. 249.—­A.  Under surface of the skull of Rhinoceros Etruscus, one-seventh of the natural size—­Pliocene, Italy.; B, Crowns of the three true molars of the upper jaw, left side, of Rhinoceros megarhinus (R.  Leptorhinus, Falconer), one-half of the natural size—­Pliocene, France. (After Falconer.)]

The Horses (Equidoe) are represented, both in Europe and America, by the three-toed Hipparions, which survive from the Miocene, but are now verging upon extinction.  For the first time, also, we meet with genuine Horses (Equus), in which each foot is provided with a single complete toe only, encased in a single broad hoof.  One of the American species of this period (the Equus excelsus) quite equalled the modern Horse in stature; and it is interesting to note the occurrence of indigenous horses in America at such a comparatively late geological epoch, seeing that this continent certainly possessed none of these animals when first discovered by the Spaniards.