The species included in this genus are A. ÆMULUS (Cope), which did not exceed the size of a Marmoset or a Red Squirrel, and had short erect incisors; A. HOMUNCULUS (Cope), a species founded on a cranium without a lower jaw, with the orbits not so large as in _Tarsius_, and the skull wide behind the eyes. "The _A. homunculus_ was nocturnal in its habits," according to Professor Cope, "and its food was like that of the smaller Lemurs of Madagascar and the Malayan islands. Its size is a little less than that of the _Tarsius tarsius_." Two other insufficiently characterised genera, both considered to be primitive Lemuroids, are _Plesiadapis_, Gervais, containing the species P. REMENSIS, P. GERVAISI, P. TOURNESARTI, and P. DAUBREI, from the Lower Eocene strata of Rheims, which have five-cusped lower molars, and enlarged upper and lower incisors; and _Protoadapis_, Lemoine, with one or two high front cusps, and a low heel to its three pre-molars; the anterior molars with two pairs of opposite cusps, the posterior molar with a fifth cusp on the hind border. P. CRASSICUSPIDENS, Lemoine, and P. RECTICUSPIDENS, Lemoine, are its two species. {119}FAMILY ADAPIDÆ. The different species associated together under this family are abundantly known from the Upper Eocene of France, England, and North America. They are remarkable in having an extra pre-molar in both jaws, the dental formula being I2/2, C1/1, P4/4, M3/3. GENUS ADAPIS. _Adapis_, Cuvier, Ossem. Foss. (2) iii., p. 265 (1822); Flower, Ann. and Mag. N. H., xvii., (1876), p. 323. _Palæolemur_, Delfort., Act. de la Soc. Linn. Bord., xxix., pp. 87-95, pl. 5 (1873); id. C. R., lxxvii., p. 64 (1873). _Aphelotherium_, Gervais, Zool. et Pal. Franç. (1), ii., Exp. 34 (1848-52). _Cænopithecus_, Rütim, Denksch. Schw. Ges. Nat., xix., p. 88 (1862). _Notharctus_, Leidy, Geol. Surv. Mont., p. 364 (1871). _? Thinolestes_, Marsh, Am. Jour. Sci., 1872 (2), p. 205. _? Telmalestes_, Marsh, _op. cit._, p. 206.