On Echinocystis and Palæodiscus—two Silurian Genera of Echinoidea

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Abstract
I. Introduction. The persistence with which the Crinoidea, Echinoidea, and Stelleroidea have maintained their essential characters, from their first appearance until the present time, has been often remarked and has received very different explanations. Thus, Prof. Poulton [ 8 , pp. 507–509], in his recent address to the Zoological Section of the British Association, deduces from the fact the vast duration of that part of the pre-Cambrian era during which the earth was inhabited. When the palæontological record begins in the Cambrian period, the phylum Echinoderma was already in existence. It was represented by the crinids, cystids, and stellerids, which were joined in the Ordovician by echinids. But, in spite of their early appearance, the oldest known members of the surviving classes may be assigned to their respective divisions as readily as if they were perfectly-preserved recent specimens. That the different classes of echinoderms have descended from a common ancestor is now unquestioned; but the search for missing links between those that still exist has not been attended with any success. The two recent classifications of the Echinoderma by Bell [ 1 ] and Häckel [ 3 , p. 164] unite the Stelleroidea and Echinoidea into one section. But the oldest known members of these two classes resemble one another less than do some of their later representatives; and we do not yet know a single intermediate form between them. Similarly, Tiarechinus , which of all Echinoidea most resembles the Crinoidea, came far too late to give any evidence as to the ancestry of either class. The

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