[Illustration: FIG. 212.—Diagram to illustrate the development of the skull and molar teeth in the camel tribe, in ascending geological order. _A_, _†Protylopus petersoni_, Uinta Eocene. _B_, _†Poëbrotherium wilsoni_, White River. (After Wortman.) _C_, _†Procamelus gracilis_, upper Miocene. (After Cope.) _D_, _Lama huanacus_, the modern Guanaco.] In the lower Eocene (Wasatch stage) lived a tiny creature, _†Trigonolestes_ (family †Trigonolestidæ), smaller even than _†Homacodon_ of the Bridger, and one of the most ancient and primitive of known artiodactyls, but, unfortunately, still represented only by very imperfect specimens, so that much which it would be highly desirable to learn must await the finding of better material. The upper molars were triangular and tritubercular, _i.e._ with three principal cusps arranged in a triangle, and are hardly to be distinguished from those of other early mammalian orders. From the teeth alone the artiodactyl nature of the animal would not have been suspected, and, in fact, they were, when first discovered, referred to primitive monkeys. The feet probably had five toes each, but this is not certain, and the femur had the third trochanter, the only known artiodactyl of which this is true. As this little Wasatch genus is so imperfectly known, it would be premature to claim it as the starting point of the camel family, and yet it may very well have been so. Better material of this genus and the links of the chain which belong in the upper Bridger and the Wind River respectively must be recovered before this earliest portion of the family history can be written in more than tentative fashion. [Illustration: FIG. 213.—Right manus of camels. _A_, _†Protylopus_, Uinta. _B_, _†Poëbrotherium_, White River. _C_, _†Procamelus_, upper Miocene. (After Cope.) _D_, Recent Guanaco.] [Illustration: FIG. 214.—Right pes of camels. _A_, _†Protylopus_. _B_, _†Poëbrotherium_. _C_, _†Procamelus_. (After Cope.) _D_, Guanaco.]