the genus Zeuglodon (fig. 228), in which the teeth differed from those of all existing forms in being of two kinds,—­the front ones being conical incisors, whilst the back teeth or molars have serrated triangular crowns, and are inserted in the jaw by two roots.  Each molar (fig. 228, A) looks as if it were composed of two separate teeth united on one side by their crowns; and it is this peculiarity which is expressed by the generic name (Gr. zeugle_, a yoke; odous, tooth).  The best-known species of the genus is the Zeuglodon cetoides of Owen, which attained a length of seventy feet.  Remains of these gigantic Whales are very common in the “Jackson Beds” of the Southern United States.  So common are they that, according to Dana, “the large vertebrae, some of them a foot and a half long and a foot in diameter, were formerly so abundant over the country, in Alabama, that they were used for making walls, or were burned to rid the fields of them.”

[Illustration:  Fig. 228.—­Zeuglodon cetoides.  A, Molar tooth of the natural size; B, Vertebra, reduced in size.  From the Middle Eocene of the United States. (After Lyell.)]

The great and important order of the Hoofed Quadrupeds (Ungulata) is represented in the Eocene by examples of both of its two principal sections—­namely, those with an uneven number of toes (one or three) on the foot (Perissodactyle Ungulates), and those with an even number of toes (two or four) to each foot (Artiodactyle Ungulates).  Amongst the Odd-toed Ungulates, the living family of the Tapirs (Tapirdoe) is represented by the genus Coryphodon of Owen.  Nearly related to the preceding are the species of Paloeotherium, which have a historical interest as being amongst the first of the Tertiary Mammals investigated by the illustrious Cuvier.  Several species of Paloeothere are known, varying greatly in size, the smallest being little bigger than a hare, whilst the largest must have equalled a good-sized horse in its dimensions.  The species of Paloeotherium appear to have agreed with the existing Tapirs in possessing a lengthened and flexible nose, which formed a short proboscis or trunk (fig. 229), suitable as an instrument for stripping off the foliage of trees—­the characters of the molar teeth showing them to have been strictly herbivorous in their habits.  They differ, however, from the Tapirs, amongst other characters, in the fact that both the fore and the hind feet possessed three toes each; whereas in the latter there are four toes on each fore-foot, and the hind-feet alone are three-toed.  The remains of Paloeotheria have been found in such abundance in certain localities as to show that these animals roamed in great herds over the fertile plains of France and the south of England during the later portion of the Eocene period.  The accompanying illustration (fig. 229) represents the notion which the great Cuvier was induced by his