The Occurrence, Structure, and Affinities of Echinocystis and Palæodiscus

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Abstract
Since their discovery about three-quarters of a century ago, Echinocystis and Palœodiscus have been the themes of an intermittent stream of publications, and, although their Echinoid nature is now generally accepted, their significance and structure are still largely problematical. Jackson (1912, p. 250) concludes his account of the ‘Order’ Echinocystoida with the only confident statement in the paragraph, to the effect that ‘further knowledge is much to be desired’. Two sets of factors, one objective and the other partly subjective, are the causes of this uncertainty. The two forms have, as yet, been found in one locality only, so that most of the material is in one state of preservation. Friable casts of flexible tests that have been crushed and telescoped before and after decalcification are ill-adapted to reveal the structure of organisms that cannot be correlated with living types; and, in the case of fossils from Leintwardine, the rare specimens that are not decalcified are often less intelligible than the others. The new material that we have collected, including specimens which, in combination, show almost every aspect of the organisms, is sufficient warrant for this addition to the literature of the two genera for that reason alone. The second factor tending to confuse our outlook on these early Echinoidea is a matter of human history and psychology. The years between 1857 (when Salter described Palœodiscus ) and 1861 (when Wyville Thomson introduced Echinocystis ) saw the publication of the ‘Origin of Species’. By 1861 all palæontologists who did not reject the

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