Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pl. 11;--31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl. 1, fig. 3;--Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl. 4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12 (not _Calymene_), (not 15, _Calymene_); pl. 28, figs. 1-5; pl. 34, fig. 1; pl. 35, fig. 7.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.--Bernard, The Apodidæ, 1892, text figs. 46, 51. _Cephalic Appendages._ No trace of antennules has yet been found. I find only three sections cut through the plane of the hypostoma of Ceraurus which show anything of the cephalic appendages, and no one of them is very satisfactory. The best is No. 22, the one figured by Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should remember that this section is not actually cut in the plane of the hypostoma but is a slice diagonally through the head, cutting through one eye and the posterior end of the hypostoma. It shows what seem to be the coxopodites of the second, third, and fourth pairs of cephalic appendages, the exopodites of the third and fourth pairs, and the metastoma. If this interpretation is correct, the first pair of gnathites lay alongside the hypostoma or under its edge, and were feebly developed, the second pair were attached in front of the tip of the hypostoma, curved back close to it, and their inner ends reached the sides of the metastoma. The third and fourth pairs were back of the metastoma, the third pair was stronger than the second, and the fourth probably like the third. [Illustration: Fig. 17. Transverse section of _Ceraurus pleurexanthemus_, showing the relation of the coxopodite to the appendifer. Traced from a photographic enlargement of the slice. Specimen 128. × 4/5.] [Illustration: Fig. 18. Slice of _Ceraurus pleurexanthemus_, showing a nearly continuous section of an endopodite and an exopodite above it. The latter is so cut as to show only the edge of the shaft and the bases of a few setæ. Traced from a photographic enlargement. Specimen in. × 4.]