The transverse furrows are the impressions left by the gnathobases of the basal joints of the legs. They were evidently long and very heavy, but the specimen has been so abraded that all details are obscured. The first six pairs of impressions are longer and deeper than the four behind. The first eight pairs seem to pertain to the thoracic appendages, while the last two belong to the pygidium. From the posterior tips of the hypostoma to the first gnathobases of which traces are present there is a distance of about 22 mm. without impressions. In _Isotelus gigas_ the hypostoma normally extends back to the posterior margin of the cephalon, so that it seems that in this specimen the impressions of the first two pairs of gnathobases under the thorax may not have been preserved. In that case, the six pairs of strong impressions may represent the last six pairs of thoracic segments, and the pygidium might begin with the first of the fainter ones. _Horizon and locality:_ From the sandstone near the base of the Aylmer (Upper Chazy) formation at Britannia, west of Ottawa, Ontario. Specimen in the Victoria Memorial Museum, Geological Survey of Canada, Ottawa. The Appendages of Triarthrus. =Triarthrus becki= Green. (Pls. 1-5; pl. 6, figs. 1-3; text figs. 1, 10, 11, 33, 42.) (Also see Part IV.) Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pl. 1, figs. 1-7;--Trans. N. Y. Acad. Sci., vol. 12, pl. 8, figs. 1-7.--Beecher, Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3;--Amer. Geol., vol. 13, 1894, pl. 3;--Amer. Jour. Sci., vol. 47, pl. 7, text fig. 1;--Amer. Geol., vol. 15, 1895, pls. 4, 5;--Ibid., vol. 16, 1895, pl. 8, figs. 12-14; pl. 10. fig. 1;--Amer. Jour. Sci., vol. 1, 1896, pl. 8; Geol. Mag., dec. 4, vol. 3, 1896, pl. 9;--Eastman-Zittel Text-book of Paleontology, vol. 1, 1900, text figs. 1267-1269;--2d ed., 1913, fig. 1375; Studies in Evolution, 1901, reprint of all previous figs.;--Amer. Jour. Sci., vol. 13, 1902, pl. 2, figs. 1-5; pl. 3, fig. 1; pl. 4, fig. 1; pl. 5, figs. 2-4;--Geol. Mag., dec. 10, vol. 9, 1902, pls. 9-11, text figs. 1-3.--Walcott, Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1 figs. 1-6;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 29, figs. 1-11; pl. 30, figs. 17-20; pl. 32; pl. 34, figs. 4-7; pl. 35, fig. 5.--Bernard, Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 11, 12.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896, text figs. 1-17, 34.--Jaekel, Zeits. d. d. geol. Gesell., vol. 53, 1901, text fig. 24. Moberg, Geol. Fören. Förhandl., vol. 29, pl. 5, 1907, pl. 4, fig. 2; pl. 5, fig. 1.--Handlirsch, Foss. Insekten, 1908, text fig. 6.--Tothill, Amer. Jour. Sci., vol. 42, 1916, p. 380, text fig. 5.--Crampton, Jour. N. Y. Entomol. Soc., vol. 24, 1917, pl.