This genus is easily distinguished, as Cope points out in his sumptuously illustrated "Vertebrata of the Tertiary Formations of the West," by the absence of the first (anterior) inferior pre-molar, and probably of the superior first pre-molar also. The canine tooth of the lower jaw is very large. The posterior pre-molar has an internal cusp, and the molars two front inner cusps. There are three species, distinguished chiefly by size, M. SPIERIANUS (Cope), very small; M. ELEGANS (Marsh), the largest, with seven teeth succeeding the canine in the lower jaw; and M. SCOTTIANUS (Cope); all from the Eocene of Wyoming. {123}GENUS HYOPSODUS. _Hyopsodus_, Leidy, Proc. Acad. Nat. Sci., Philad., 1870, p. 109. The present genus is recognised by the front inner cusp of the lower molars being single, and their heel presenting a cusp at its inner hind angle (except in _H. acolytus_). Of the upper pre-molars, the median and posterior have an internal cusp; and the molars have two outer and two inner cusps with two small intermediate tubercles. There are six species known, from the Wasatch and Bridger beds of Wyoming and New Mexico, of which H. ACOLYTUS is distinguished by having the heel of the anterior and median lower molars without an inner hind cusp. Professor Cope remarks that though the species of this genus are not numerous, individuals of some of them are exceedingly common in the Eocene beds of Wyoming. H. PAULUS and H. MINUSCULUS, Leidy, H. VICARIUS and H. POWELLIANUS, Cope, with H. JURENSIS, Rutimeyer, from the Upper Eocene of Egerkingen, are the best known species. The genera INDRODON, Cope, from the Lower Eocene Puerco formation of New Mexico, with three cusped upper and four cusped lower molars; OPISTHOTOMUS, APHELISCUS, and SARCOLEMUR, Cope, from the Wasatch of Wyoming; HIPPOSYUS, Leidy; BATHRODON, MESACODON, and STENACODON, Marsh, from the Middle Eocene Bridger beds; are of doubtful affinities. II. THE MONKEYS AND APES--SUB-ORDER ANTHROPOIDEA.