Fig. 6. Specimen 219. An entire specimen of medium size, developed from the ventral side. It shows particularly well the "normal" curvature of the antennules, the change in form of the segments of the endopodites from cephalon to pygidium, and, along the axial lobe, the apodemes of the ventral integument. See also pl. 4, fig. 4. × 3.6. PLATE II. HELIOTYPE CO. BOSTON * * * * * PLATE 3. Photographs of _Triarthrus becki_, made by C. E. Beecher. Fig. 1. Specimen 204. See also text fig. 42 and pl. 4, fig. 6. The exopodites and endopodites of the first few segments of this specimen are better preserved than those of any other revealing them from the dorsal side, × 9.5. Fig. 2. Specimen 220. A large individual exposed from the lower side. It shows well the endopodites and part of the exopodites, and, rather better than any other specimen, the endobases of the coxopodites. × 2.4. Fig. 3. Specimen 216. A small entire specimen showing considerable of the detail of the appendages of the cephalon, and some of those of the remainder of the body, × 7.4. Fig. 4. Specimen 201. This figure shows the details of the appendages of the left side and of the pygidium. Note the plate on the median line back of the pygidium, the sockets for spines, and the terminal spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4, figs. 1, 2. × 7.1. Fig. 5. Specimen 207. One half of the posterior part of the thorax and pygidium, showing exopodites and endopodites as seen from the dorsal side, × 7.6. Fig. 6. Specimen 214. The exopodites have been turned back nearly parallel to the axis of the shell. Notice particularly the long flattened setæ and the spinose spatula-shaped terminal portion of each shaft. See also pl. 1, fig. 2. × 7. PLATE III. HELIOTYPE CO. BOSTON * * * * * PLATE 4. Photographs of _Triarthrus becki_, made by C. E. Beecher. Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3, but showing more clearly some details of spines on the endopodites. × 12.66. Fig. 2. Specimen 201. Three appendages on the right side of the thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. × 12.66. Fig. 3. Specimen 223. A small crushed specimen which nevertheless shows well the appendages of the right side of the thorax, developed from the ventral side. Note coxopodites, exopodites, and endopodites, and that all appendages are moved equally laterally from their original position. × 11.4. Fig. 4. Specimen 219. Another photograph, with different lighting, of the individual shown in pl. 2, fig. 6. This print brings out better the coxopodites and the folds of the ventral membrane. × 3.23.