Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1, 3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1 (restoration), 2;--Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8, figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs. 1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_), 13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1 (restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f. Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32; pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796, fig. 12.--Bernard, The Apodidæ, 1892, text figs. 50, 52, 54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13, 15, 17.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896, fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7. In both of Walcott's accounts (1881, 1918) of the appendages of _Calymene_ and _Ceraurus_, he has described them together, so that those who have not taken time to study the illustrations and disentangle the descriptions are very apt to have a confused notion in regard to them. I have therefore selected from the original specimens those slices of _Calymene_ which are most instructive, and bearing in mind the probable appearance of the appendages of an enrolled _Triarthrus_, have tried to interpret them. In such a method of study, I have of course started with a pre-formed theory of what to expect, but have tried to look for differences as well as likenesses. _Cephalic Appendages._ _Antennules._--The evidence of antennules rests on a single slice (No. 78). The appendage in question is exceedingly slender and arises at the side of the hypostoma near its posterior end. It shows fine, slender segments, and curves first outward and then forward. If it is in its natural position, it is not an antennule, but the endopodite of the second or third pair of cephalic appendages. It is short, only about one-third the length of the hypostoma, but is doubtless incomplete. The two distal segments show a darker filling, indicating that they were hollow. Judging from analogy with other trilobites, the appendage is probably an endopodite and not an antennule. There can be no reasonable doubt, however, that _Calymene_ possessed antennules.