1871.--On the structure of trilobites. Geol. Mag., dec. 1, vol. 8, pp. 289-294, pl. 8. 1884.--Notes on the appendages of trilobites. Geol. Mag., dec. 3, vol. 1, pp. 162-165, 2 text figs. 1895.--Some points in the life history of the Crustacea in early Palæozoic times. Quart. Jour. Geol. Soc., London, vol. 51, pp. lxx-lxxxviii, 1 pl. * * * * * PLATE 1. Photographs of _Triarthrus becki_, made by C. E. Beecher. Fig. 1. Specimen 213. The dorsal test has been removed from the glabella, revealing the outline of the posterior end of the hypostoma, the proximal ends of the antennules, the gnathites, and incomplete endopodites of some appendages, × 5.43. Fig. 2. Specimen 214. The head of a complete large specimen. Part of the thorax is shown on pl. 3, fig. 6. Note especially the form of the segments of the endopodites and of the anterior coxopodite on the right side, × 7.33. Fig. 3. Specimen 217. This specimen shows better than any other the form of the gnathites of the cephalon. Note also the setæ of the exopodites under the cheek at the right. The appearance of a hook on the posterior gnathite on the right may be accidental, but it does not show broken edges, × 6.85. Fig. 4. Specimen 215. The ventral side of the cephalon of a small entire specimen. Shows well the form of some of the gnathites and a few of the endopodites. Note the unusual position of the antennules. × 7.63. Fig. 5. Specimen 226. This specimen did not photograph well, but is important as showing the exopodites and endopodites emerging from under the cephalon. × about 6. PLATE I. HELIOTYPE CO. BOSTON * * * * * PLATE 2. Photographs of _Triarthrus becki_, made by C. E. Beecher. Fig. 1. Specimen 201. The entire specimen, details of which are shown in pl. 3, fig. 4 and pl. 4, figs. 1, 2. The dorsal test has been removed from the anterior segments on the right side. × 4.12. Fig. 2. Specimen 206. A small individual with the endopodites, and the exopodites minus their setæ; well preserved on the left side. Note the position of the antennules. The course of the facial suture is unusually well shown. × 10. Fig. 3. Specimen 210. The specimen which served as the main basis for Professor Beecher's first figure of the appendages of the thorax, specimen 206 (fig. 2, this plate) having supplemented it. Note the "normal" position of the antennules and the extension of the appendages from beneath the pleural lobe. Specimens with the antennules in this position may possibly be males. × 4. Fig. 4. Specimen 205. A small specimen with some of the appendages preserved, especially toward the posterior end, but particularly valuable for the unusually well preserved metastoma. × 11. Fig. 5. Specimen 211. A small cephalon, cleaned from the ventral side, and showing well the gnathites which approach each other unusually closely on the median line. × 10.5.