When both endopodites and the "spiral branchiæ" are present in the same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiæ" are dorsal to the endopodites, as the setæ of the exopodites would be expected to be. The specimens which show the clear spots connected, and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to bear evidence against this interpretation, but one has only to think of the effect of cutting a section along the edge where the setæ are attached to the shaft of the exopodite of _Triarthrus_ to see that such a zig-zag effect is entirely possible. One would expect to cut just this position only rarely, and, in fact, the zig-zags are seen in only three or four sections. The bifurcation of the basal segment of the "spiral branchiæ" (pl. 3, fig. 10, 1881) is probably more apparent than real, if indeed these basal segments have anything to do with the succeeding one. A second peculiarity of _Calymene_, shown in Walcott's restoration, is the great enlargement of the coxopodites and of the distal segments of the endopodites of the fifth pair of appendages of the cephalon. This is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881). After a study of the specimens I regret to find myself still unconvinced that the posterior cephalic appendages were any larger than those in front. Ventral Membrane. The most striking value of the thin sections of _Ceraurus_ and _Calymene_, and therein they have a great superiority over all the other forms so far investigated, is that they show the extent of the body cavity and the position, though not the substance, of the ventral membrane. Transverse sections through _Ceraurus_ (Walcott's pl. 1. figs. 1-5; pl. 2, figs. 1, 3, 1881) and _Calymene_ (pl. 3, figs. 9, 10, 1881) show that the body cavity was almost entirely confined to the axial lobe. The longitudinal sections of _Ceraurus_ (pl. 2, figs. 6, 8; pl. 4, fig. 8) and of _Calymene_ (pl. 2, figs. 5, 7; pl. 5, figs. 1-4) show that the ventral membrane was exceedingly thin and was wrinkled transversely when the shell was enrolled.