In the following studies of the several families of the Palæechinoidea it is the intention to take up for consideration the several genera and species in their natural systematic order of sequence as expressed in the proposed new classification (see table facing page 242). This intention is carried out, as far as present knowledge admits, in families succeeding the Melonitidse. In the present family of the Melonitidse, however, the rule is departed from, because the genera and species of which fullest knowledge is attained are the more specialized, and the more primitive genera are least known as far as available material goes. In this family, therefore, the genera and species are taken up in the order of convenience for handling the several cases rather than in the proper systematic order. The natural systematic sequence of the genera would be, Rhoechinus, Palæechinus, Oligoporus and Melonites, as . . .