latter family. The latter only differs from the former in the increased complexity of the premolars. The passage to the horse line is suggested by the resemblance of Pliolophus

FIG. 8. Systemodon tapirinus Cope, dentition; upper figure, right superior molars from below; lower figure, left inferior molars from above; natural size. Original; from Wasatch bed of Wyoming.

AOODS

FIG. 9. Heptodon ventorum Cope, superior molars of left side, natural size. Original; from Wind River beds of Wyoming. From "Report U. S. Geol. Surv. Terrs.," F. V. Hayden, vol. iii.

(= Orohippus Marsh) to Anchitherium of the family of the Palæotheriidæ. The transition is seen in some genera of the intermediate family of the Chalicotheriidæ, beginning with Ectocium (Cope).

FIG. 10. Hyrachyus agrarius Leidy, superior molar teeth, natural size; from Bridger bed of Wyoming. From Leidy, "Report U. S. Geol. Surv. Terrs.," F. V. Hayden, vol. i.

Three species of Systemodon are known, all from the Wasatch Eocene. The largest, S. tapirinus Cope, was equal to a large ram. A dozen species of Hyracotherium are known from the Eocene of Europe and North America, which range in size

from that of a sheep to that of a kit-fox. Several species of Helaletes have been described from the Bridger Eocene. The species of Hyrachyus are rather numerous, and range through

FIG. 11. Lophiodon isselensis, superior molars three-fifths natural size; from Middle Eocene of France. From Gaudry, "Enchainements du Régne Animal." the Wind River, Bridger, and Diplacodon beds. Dilophodon has two species and Colonoceras one species, in the Bridger. Lophiodon is, so far as yet known, confined to Europe. One of its species, L. rhinoceroides, was as large as a medium-sized rhinoceros, and several species ranged in dimensions to those of a sheep. They appear in the Bruxellian, and range upwards through the Bartonian. Heptodon is, so far, only American; two species, a large and a small, are found in the Wasatch formation, and two, of intermediate size, in the Wind River.

FIG. 12. Protapirus priscus Filh., Phosporites of Quercy, France, three-fourths natural size. From Filhol.

Protapirus is from the Phosphorites of France; and Isecto

PLATE XXXI.

P

Hyrachyus agrestis, Leidy, natural size; from Bridger bed of Wyoming. Unshaded portions not preserved. Original, from specimen in Coll. E. D. Cope.

lophus is from the summit of the Eocene in North America, or from the Diplacodon beds.

As but one genus of TRIPLOPODIDE is known, its origin and probable descendants only can be referred to. It may very readily have been derived from Heptodon, with which it agrees in dental formula. The loss of the heel of the last inferior molar and of the fifth anterior digit would metamorphose Heptodon into Triplopus. Two species are known, both from the upper division of the Bridger Eocene. A possible third species is from the Diplacodon bed (Prothyracodon S. and O.).

FIG. 13. Skull of Triplopus cubitalis Cope, from the Bridger Eocene of Wyoming, three-quarters natural size; right side. Original; from the "Report U. S. Geol. Surv. Terrs.," vol. iii. Fig. a, superior molars with deciduous premolars; b, third and fourth superior premolars.

The CENOPIDAE include genera which have lost the superior canine teeth, and have thus come to resemble the rhinoceroses. The known genera resemble these animals also in the reduction in the number of the incisors, and in one genus at least a dermal horn was present. The two genera known differ as follows:

No dermal horn;.....

A dermal frontal horn;

Canopus Cope.
.Dihoplus Brandt.

This family constitutes the transition between the Lophiodontine group of the Lophiodontidæ by Heptodon or Hyrachyus,