Book Description

Excerpt from The Geological Magazine, or Monthly Journal of Geology, 1918, Vol. 5: With Which Is Incorporated the Geologist, Nos. 643 to 654; Decade Vi The primitive and ccnservative nature of the Holectypoida' among Irregular Echinoids has been so Often indicated, and indeed is so manifest, that it may be taken for granted without further argument. It follows from this that there are but two alternative possibilities for the lines of descent of all other Orders of the sub-class. Either they must have arisen from some Regular stock independently of the Holectypoida, in which case they need show no special resemblances to that group save under the influence of parallelism of development or else they must have descended directly or indirectly from Holectypoid ancestors. 'in the present paper a brief description of the peristomial characters of certain representative Irregular Echinoids is given, and an attempt is made to indicate the presence or absence of relationship to the Holectypoida shown by this fragment of evidence. The genera whose internal test-structure has been examined for this purpose are C'lypeaster, Echz'nooyamus, and Echin amchm'us among the Clypeastroida, qcleolz'tes and Trematopygus among the Nucleolitoida, Conulopsz's' among the ?cassiduloida, and Eol[znocorys, Me'omster, and Ecln'nocardz'um among the Spatangoida. In the case of the first and last mentioned Orders, the genera studied may be taken as fairly representative of the groups to which they belong; but the other 'two Orders (usually grouped together under the name here restricted to the latter) are very imperfectly illustrated. This is accounted for by the extreme difficulty of the preparation of the Jurassic forms and the virtual impossibility, under existing circumstances, of acquiring suitable material of the Casse'dulus Eclzz'nolampas series, which are almost unrepresented in the British fauna, both past and present. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.