_Measurements:_ Total length of specimen, 109 mm. Probable length when complete, 116 mm. Length of cephalon, 40 mm.; width at genal angles, restored, about 62 mm. (Billings' restoration). Width of doublure of front of cephalon on median line, 17 mm.; length of hypostoma, 20 mm. Length of coxopodite of last appendage on left side of cephalon, 10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter of coxopodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of coxopodite on left side of the second segment of the thorax, 11 mm.; diameter, about 2.5 mm. Length of basipodite of the same, 5 mm.; diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter, about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than the total length as the segment is not completely exposed.) Distance between proximal ends of gnathobases of the fifth thoracic segment, about 7 mm. Distance between outer ends of the coxopodites of the first thoracic segment (estimated from measurements on the left side), 27 mm Distance apart of the dorsal furrows at the first thoracic segment, 27 mm. Length of the longest exopodite which can be traced, about 20 mm. =Isotelus maximus= Locke. (pl. 10, fig. 2.) Illustrated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol. 6, 1883, p. 200, figs. 1-3 (endopodites and coxopodites). Walcott, Science, vol. 3, 1884, p. 279, fig. 1 (endopodites, coxopodites, and traces of exopodites). Woodward, Geol. Mag., dec. 3, vol. 1, 1884, p. 162, figs. 1-3 (copies of Mickleborough's figures). Bernard, The Apodidæ, 1892, text fig. 49. Beecher, Amer. Jour. Sci., vol. 13, 1902, p. 169, pl. 5. figs. 5, 6 (outline from one of Mickleborough's figures and an original figure). Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 133, pl. 24, figs. 3, 3a; pl. 25, fig. 1. This specimen, which conies from the Richmond strata 2 miles north of Oxford, Ohio, is the best preserved of the specimens of _Isotelus_ with appendages which has so far been found. The individual consists of two parts, the actual specimen, and the impression of the ventral side. To describe it I am using very skillfully made plaster reproductions of both parts, presented to the Museum of Comparative Zoology by Doctor Charles D. Walcott, and presumably made after he cleaned the specimen as described in Science (1884). I have also an enlarged photograph (pl. 10, fig. 2) which seems to have been made after some later period of cleaning, probably by Professor Beecher, and I have examined the original specimens in Washington.