There have now been described all the known members of the Edrioasteridae and the very similar genus Stegarioblastus, as well as a genus apparently connected with the Agelacrinidae but presenting some remarkable features, namely Pyrgoeystis. It is proposed in this and the following Study to deal particularly with the Edrioasterid organization, first considering what the known facts of skeletal structure imply as to the general anatomy and mode of life, secondly comparing the structure with that of other echinoderms, especially the Asteroidea. Although it may be necessary to refer to various facts that fall to be dealt with in future Studies, still the present seems a convenient opportunity for this general discussion, because of the recent publication by Mr. W. K. Spencer of the introduction to “A Monograph of the British Palaeozoic Asterozoa” (Feb., 1914, Palæontogr. Soc. vol. for 1913), by Dr. J. F. Gemmill of his highly important memoirs on the starfishes Solaster endeca (1911, Proc. R. Phys. Soc. Edinburgh, vol. 18, pp. 174-91 ; and Feb., 1912, Trans. Zool. Soc. London, vol. 20, pp. 1-71, pis. i-v) and Asterias rubens (Oct., 1914, Phil. Trans., ser. B., vol. 205, pp. 213-94, pis. xviii-xxiv, and March, 1915, Proc. Zool. Soc. London, pp. 1-19, pis. i-iii), and by Dr. A. F. Foerste of some suggestive “ Notes on Agelacrinidæ” (Sept., 1914, Bull. Sci. Lab. Denison Univ., vol. 17, pp. 399-486, pis. i-vi).