THE recent discovery that the crinoids in their ontogeny increase the number of their anibulacral post-radial ossicles by the interpolation of numerous ossicles (in pairs) between the first two primitive brachials and the radial, forming what are known as "interpolated division series, " as well as by the addition of brachials in a linear series at the growing tip of the aril, where only heretofore addition to the number of the brachials was supposed to occur,' has shown that in the manner of increase of the number of ambulacral segments there is a close similarity between the crinoids and the echinoids, both groups adding new plates between those already formed and the radials (oculars), whereas in the ophiuroids and the asteroids new plates are added only at the tip of the arms, not, however, at the extreme tip, as in the crinoid arm, but just proximal to a permanent plate; and the question naturally arises, can the commonly accepted view regarding the interrelations of the various classes of the Echinodermata, be maintained in the light of the present state of our knowledge? So long ago as 1821 J. S. Miller remarked on the similarity of an inverted Cidaris to a crinoid, and this similarity was also noticed by Loven. That this similarity is not superficial but in reality fundamental has become increasingly evident to me during the course of my studies on the echinodernms, and I have now no hesitation in stating that the crinoids and the eclinoids have much more in common, and are much more closely related to each other, than either group is to the a-steroids or the ophiuroids. Considering only the external skeleton, we find that, in the crinoids and echinoids (1) the .ambula-crals in-