OCR Text |
Show 1000 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, separated by small but well-defined interbrachials, while a few interdistichals are also present; interbrachials also occur in Apio-crinus roissyanus and A. elegans. In both these forms, however, the plates of the cup are too thick, and the arms too much branched, for us to infer any direct affinity with Uintacrinus. I refer to these facts merely to show the possibility of a development of interbrachials and interdistichals among the genera of the group. Similarly the Pentacrinidae, notably Extracrinus, may develop interbrachials; and in E. lepidotus, at any rate, the proximal pinnules seem to have been laterally united, forming thin scaly plates. Moreover the whole arm-structure of Uintacrinus is singularly like that of the Pentacrinidae. But we must look for a form not so highly modified, and one in which the arrangement of syzygies and pinnules does not conflict with the law of their arrangement in Uintacrinus. Thus, by a gradual process of elimination, w e are forced back upon Dadocrinus, and here indeed appears to be the object of our search. The genus Dadocrinus, based by H . von Meyer on Encrinus gracilis, von Buch, which has been investigated by Beyrich, Kunisch, Giiricb, Wachsmuth and Springer, and above all by A. von Koenen \ is admitted to be an ally of Erisocrinus, Stemmato-crinus, and Encrinus, and to be intermediate between them and the Pentacrinidae and Apiocrinidae. Whether or no it be the actual ancestor of the latter families, it at least comes as near as possible to what that ancestor must have been. It is pseudo-monocyclic, has 2 primibrachs, pinnulate arms bifurcating once, wdth a separate axial canal; its primibrachs are united by a number of small interbrachials which pass up into a thin-plated flexible tegmen. The arrangement of pinnules and syzygies in the proximal region of tbe arms is governed by the same law, and subject to the same regular exceptions, as in Uintacrinus (fig. 13)2. The proximal pinnules are larger than the others, and have flat backs and square sides, thus resembling the coadunate pinnules of Extracrinus. N o w in all these essential points of structure, Dadocrinus gradlis agrees precisely wdth Uintacrinus. All that w e have to suppose is a gradual exaggeration of these features and the loss of the stem. The former is a natural supposition, since, as has just been shown, 1 "Beitrag zur Kenntniss der Crinoiden des Muschelkalks," Abh. Ges. Wiss. Gottingen, xxxiv. Phys. Kl. i. pp. 1-44, pi. i. (1887), and " Ueber dieEnt-wiekhing von Badocrinus gracilis, v. Bucb, und Holocrinus wagneri, Ben., und ihre Verwandtscbaft mit an deren Crinoiden," Nacbricbten Ges. Wiss. Gottingen, Math.-pbys. Kl. Jahrg. 1895, pp. 283-293 (pages 292, 293 being wrongly imposed), 14th Dec, 1895. In these papers references to the rest of tbe literature will be found. 2 Von Koenen (op. cit. 1895) describes tbe pinnules as borne on the outer side of IlBr2, on the inner side of IIBr4, and thence regularly on each secundi-braeh, on the outer and inner sides alternately. The syzygies that are to be inferred from tbis arrangement agree with tbe plan of Uintacrinus. But I have found yet other arrangements, which make tbe resemblance still more strikirg. A slab in tbe British Museum (E 6070) enables one to trace the arrangement in several arms. rlhe commonest type has syzygies between secundibrachs.l & 2, 3 & 4, 6 & 7 ; pinnules are borne by the epizygals and by HBr5, first on the |