The first mention of _Neolenus_ with appendages preserved was in Doctor Walcott's paper of 1911, in which two figures were given to show the form of the exopodites in comparison with the branchiæ of the eurypterid-like _Sidneyia_. In 1912, two more figures were presented, showing the antennules, exopodites, and cerci. The specimens were found in the Burgess shale (Middle Cambrian) near Field, in British Columbia. This shale is exceedingly fine-grained, and has yielded a very large fauna of beautifully preserved fossils, either unknown or extraordinarily rare elsewhere. It was stated in this paper (1912 A) that trilobites, with the exception of _Agnostus_ and _Microdiscus_, were not abundant in the shale. In discussing the origin of the tracks known as _Protichnites_, Walcott presented four figures of _Neolenus_ with appendages, and described the three claw-like spines at the tip of each endopodite. Three new figures of the appendages were also contributed to the second edition of the Eastman-Zittel "Text-book of Paleontology" (1913, p. 701). Later (1916, pl. 9) there was published a photograph of a wonderful slab, bearing on its surface numerous Middle Cambrian Crustacea. Several of the specimens of _Neolenus_ showed appendages. Finally, in 1918, appeared the "Appendages of Trilobites," in which the limbs of _Neolenus_ were fully described and figured (p. 126), and a restoration presented. Organs previously unknown in trilobites, epipodites and exites, attached to the coxopodites, were found. =Neolenus serratus= (Rominger). (Text fig. 2-8.) Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20, pl. 6, figs. 1, 2 (exopodites of thorax and cephalon);--Ibid., vol. 57, 1912, p. 191, pl. 24, figs. 1, la (antennules, caudal rami, and endopodites of thorax);--Ibid., vol. 57, 1912, p. 277, pl. 45, figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal rami);--Text-book of Paleontology, edited by C. R. Eastman, 2d ed., vol. 1. 1913, p. 701, fig. 1343 (exopodites), p. 716, fig. 1376 (abdominal appendages), fig. 1377 (appendages of thorax and pygidium);--Ann. Rept. Smithson. Inst. for 1915, 1916, pl. 9;--Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pl. 14, fig. 1; pls. 15-20; pl. 21, fig. 6; pls. 22, 23; pl. 31 (restoration); pl. 34, fig. 3 (restored section); pl. 35, fig. 4; pl. 36, fig. 3 (hypostoma). The following description of the appendages of _Neolenus_ is summarized from Walcott's paper of 1918, and from a study of the eight specimens mentioned below. _Cephalon._