_Lithocyst and Concretion._--The cavity filled by the concretion is lined in places by a single layer of cells, two of which are shown in Fig. 7. This fact has been noted by both H. V. Wilson and Conant. Such cells are evidently remnants of the cells that formed the concretion. The supporting lamella completely surrounds the cavity of the concretion. The concretion filling the lithocyst has the shape of a hemiprolate spheroid cut in the plane of the axis of revolution. Whether it is of endo- or of ectodermal origin, I believe developmental studies only can determine. Tests made in the Chemical Laboratory show the presence of calcium sulphate with perhaps a very small trace of phosphate.[i] Nitric acid slowly dissolves these concretions, but I believe Claus was mistaken when he said that they dissolve with an evolution of gas. I watched them dissolve under the microscope, and never could see the least bit of gas formed. If Claus’s observation is correct, then the composition of the concretions of C. marsupialis is different from that of the concretions of C. Xaymacana. The concretions, further, were dissolved out of the material preserved in formaline and in osmic acid solutions. For dissolving them in situ I used either nitric or hydrochloric acid, or both. A slight husk remains after all the lime is dissolved. _The Epithelium of the Clubs._--The epithelium is thickest on the dorsal side of a club. The thickening here, as in several other places, seems to be due to a crowding of the cells, in consequence of which the nuclei come to lie at different levels, but I believe that all the cells quite reach the surface. The cells with their nuclei nearest the surface are pyramidal in shape, with the bases of the pyramids toward the surface, while those cells whose nuclei lie deeper (where several layers of nuclei occur) may be spindle-shaped (Figs. 12, 23, 24, 26). Centrad these cells are continued into a single process, which often seems to extend to the basement membrane (Figs. 7, 12, 13, 23, 24). Where the epithelium covers the region of the concretion, the cells become flattened and with the long axis of their nuclei parallel with the surface of the club (Fig. 7). The same holds true for the corneal epithelium (Figs. 7, 13). It is a significant fact that in many places the nuclei form only a single layer, and in such places one cannot speak of spindle-shaped cells. I cannot find any evidence of sensory and supporting cells as Schewiakoff describes. The fact that spindle-shaped cells may exist is simply a physical consequence of their being closely crowded. Conant arrived at the same conclusion.