This genus contains but one species, founded on several lower jaws disinterred from the Wasatch beds in the Big-Horn Bad-lands, in Northern Wyoming. The lower incisors, or perhaps, canines, are very large and close to the line of union of the two halves of the jaw; the molars have three cusps in front and a heel behind. The dental characters of the genus "resemble considerably those of _Anaptomorphus_ and _Necrolemur_ [_Microchærus_] but the large size of the inferior canine {117}or incisor tooth distinguishes it from both." (Cope.) C. LATIDENS, Cope, is the only species. GENUS OMOMYS. _Omomys_, Leidy, Journ. Acad. Nat. Sci. Philad., vii., p. 408 (1869). This genus was established for the first Mammalian fossil--a lower jaw--described from the Bridger-beds as O. CARTERI. The posterior lower molar has cusps in opposing pairs; pre-molars, three in number, the two anterior one-cusped, the posterior two-cusped. The chin was longer and less rounded than in _Anaptomorphus_. GENUS ANAPTOMORPHUS. _Anaptomorphus_, Cope, Proc. Amer. Phil. Soc., 1872, p. 554; id., Rep. U. S. Geol. Surv., iii., p. 245, pl. xxiv. e, fig. 1; xxv., fig. 10. This genus was founded by Cope on an almost entire cranium discovered in the Bridger (Eocene) beds of the upper Valley of Green river, and on other remains from what is known as the Wasatch formation of the Big-Horn Basin in Wyoming Territory, in North America. The external upper incisor is small and set close to the small canine; the pre-molars have each a large external and a smaller internal cusp; the true molars are wide and have one internal and two external cusps. In the lower jaw the two anterior molars are four-cusped, with a transverse ridge between the anterior pair, and an oblique ridge between the hind inner, and the front outer, cusp; the posterior is three-cusped and has a heel. The orbits are enclosed, as in typical Lemurs. Not less typical characters are the position of the lachrymal foramen, external to the orbit, and the unossified halves of the lower jaw. "Its dental formula (I2/2, {118}C1/1, P2/2, M3/3) agrees only with the _Indrisinæ_. But no known _Lemuridæ_ possess anterior lobes and cusps on all the pre-molars, so that in this respect, as in the number of its teeth, this genus resembles the higher Monkeys, the _Simiidæ_ and _Hominidæ_, more than any existing member of the family.... It has ... a number of resemblances to _Tarsius_, which is, perhaps, its nearest ally among the Lemurs, although that genus has three pre-molars.... There is no doubt but that the genus _Anaptomorphus_ is the most Simian Lemur yet discovered...." (_Cope._)