Two rival hypotheses as to the origin of the Echinoidea are prevalent to-day. One of them postulates as the primitive form an organism built of a few large plates arranged in vertical series; the other demands that the earliest types should have been “scaly”, with many small plates which were regular only along the radii. Those who hold the former hypothesis see in the Ordovician Bothriocidaris a close approximation to their ideal; supporters of the latter claim that the Silurian genera Echinocystis and Palaeo1discus represent fairly well the ancestral qualities of the class. So long as Bothriocidaris is regarded as a true Echinoid, the Silurian forms lose much of their claim to be considered truly primitive; the chronological sequence is against them.