. The Crinoidea flexibilia (with an atlas of and 76 plates). Crinoidea, Fossil. CLASSIFICATION 117 lent of the Kaskaskia into the Lower Coal Measures, where it has a character- istic representative in Ampliicrinus; in this, however, there is exhibited a tendency to reversion to the structure of the weak anal side. This group may be subdivided into three families, the first two of which are more sharply differentiated from the third than they are from each other. The character upon which the principal distinction is based is found in the condition of the infrabasals, and its importance m
. The Crinoidea flexibilia (with an atlas of and 76 plates). Crinoidea, Fossil. CLASSIFICATION 117 lent of the Kaskaskia into the Lower Coal Measures, where it has a character- istic representative in Ampliicrinus; in this, however, there is exhibited a tendency to reversion to the structure of the weak anal side. This group may be subdivided into three families, the first two of which are more sharply differentiated from the third than they are from each other. The character upon which the principal distinction is based is found in the condition of the infrabasals, and its importance may be made clear by reference to the growth of the calyx plates in the young comatulids. Leaving out of consid- eration the infrabasals in the latter as usually too rudimentary for comparison, the calyx cup in the early stages consists of basals and radials, both of which form an important part of its wall; the basals are erect, and help to enclose the visceral mass. This is well illustrated in the young Antedon as worked out by W. B. Carpenter, and in the larval Comactinia herein shown on Plates B and C. The basals at first are not very far from parallel with the dorso-ventral axis of the animal. At a later stage the basals undergo considerable diminution in their external size, and become more and more recumbent in position until they have slipped inward under the central part of the dorsal cup, disappearing from sight altogether; at this stage they cease to take any essential part in the' struc- ture of the calyx wall, and in some cases lose their identity by fusing to form the so-called rosette. Primitively the infrabasals underwent a similar trans- formation, and in the comatulids have been quite lost. This is more fully explained by Mr. Austin H. Clark (Proc. U. S. National Museum, vol. 38, pp. 117, 118), from whose language I have borrowed freely; and he finds that among the Recent crinoids the best characters for the differentiation of the higher groups are found in the
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