This family includes certain fossil forms of Lower Eocene age from the phosphatic deposits of Quercy in France, the {115}Wasatch strata of Wyoming, and the Puerco beds in New Mexico. Their dental formula is the same as that of existing Lemurs, namely I2/(2-1), C1/(1-0), P(2-3)/(2-3), M3/3. In some of the genera there is a tendency to develop, as Cope has pointed out, large cutting teeth in the position of incisors, "thus approaching the Aye-Aye." The posterior pre-molars are more simple than the anterior true molar, a character which indicates some relationship to the Mouse-Lemurs (_Chirogale_). The mastoidal or posterior portion of the ear-capsules, and the neighbouring squamosal region of the cranium are swollen, as among the Galagos. GENUS MICROCHÆRUS. _Microchærus_, Wood, Lond. Geol. Journ., i., p. 5 (1846). _Heterohyus_, Gerv., Zool. et Pal. Fr., p. 202, pl. 35, fig. 14. _Necrolemur_, Filhol, C. R., lxxxvii., p. 1112 (1873); id. Ann. Sc. Geol., viii., p. 55, pl. iv., figs. 213-217 (1877). This genus is distinguished from all other Lemurs by "the angle of the mandible being produced into a large hook-like flange." (Flower and Lydekker.) The orbits are large, indicating a nocturnal animal; the inter-orbital space is wide, and distinguishes it from _Loris_. The dental formula is I2/1, C1/1, P3/3, M3/3. The canine teeth are not prominent; the anterior lower pre-molar is only slightly developed; a gap separates the anterior and the median upper pre-molars. This genus is represented by five species. MICROCHÆRUS ANTIQUUS (Filhol) is of very small size, and has many affinities with _Galago_, as exhibited in the well-preserved cranium that has been recovered from the Phosphorites of Central France. The two lower molars have only one root. M. ERINACEUS, {116}Wood, from the Upper Eocene of Hampshire; M. EDWARDSI (Filhol), from Central France, a species larger than _M. antiquus_, presents dental characters similar to the Galagos and the Mouse-Lemurs; M. PARVULUS (Filhol), and M. ZITTELI (Schlosser), are both from the Quercy Phosphorites of France; while M. ARMATUS is from the Eocene of Alsace, and M. (CRYPTOPITHECUS) SIDEROLITHICUS from the Bonerg of Frohnstellen. GENUS MIXODECTES. _Mixodectes_, Cope, Proc. Amer. Phil. Soc., p. 447 (1883); id., Rep. U. S. Geol. Surv., iii., p. 240, pl. xxiv. f, figs. 1 and 2. The members of this genus, founded on fragmentary mandibles from the Puerco (Lower Eocene) strata of New Mexico, have a large front tooth "issuing from the ramus at the symphysis like a rodent incisor, the second tooth being similar but smaller and posterior and external to the first." The genus is represented by two species, M. PUNGENS, Cope, and M. CRASSIUSCULUS, Cope. GENUS CYNODONTOMYS. _Cynodontomys_, Cope, Palæont. Bull., p. 151 (1882); id., Rep. U. S. Geol. Surv., iii., p. 243, pl. xxiv., fig. 2.