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Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 997 both test and arms, are obvious points that have already been alluded to. It is also noteworthy that each of these very differently constituted cups resembles the others in one curious feature, namely, the presence of a central, pentagonal, apical plate. One may say, if one chooses, that in Saccocoma, this represents the fused basals, and in Uintacrinus the fused infra-basals; but in Marsupites it must be something else. Or one may say that in each case it is the same element, be it the proximal stem-ossicle (which some erroneously call " centrodorsal"), or the distal stem-ossicle (which some, seeking an homology, have called " dorso-central"), or perhaps a new plate altogether, a simple supplementary plate developed to fill up the gap left by the disappearance of the stem. One might argue for ever : there is no evidence. The one obvious fact is that such a central plate is found in three very different forms, all of which were free-swimming, and unlike all other crinoids in showing absolutely no trace of a stem. It is therefore not safe to ascribe to the central plate any morphological significance, or to give it any name other than " centrale." The large size of the body is produced in Marsupites, and still more in Saccocoma, by the largeness of the plates; in Uintacrinus it is produced by the incorporation of many brachials, interbrachials, &c. into the dorsal cup. The large size being probably a secondary character, it is not fair to argue that tbe ancestors of Uintacrinus had so many plates in the dorsal cup ; although we must infer that they were forms that had a tendency to this mode of enlargement of the cup. The essentials of structure in Uintacrinus appear thus to be : 5 basals ; 5 radials ; 5 arms, branching once ; the two primibrachs, at least, united by interbrachial; pinnules borne by secundibrachs, beginning with IIBr2; a tendency for proximal pinnules to coalesce ; an axial canal separated from the ventral groove of the brachials. As claimants to provide an ancestor for Uintacrinus the Camerata, notwithstanding the superficial resemblance in the cup of many of them to this genus, must at once be put out of court as having no separate axial canal in the arms. The flexible tegmen of Uintacrinus is also removed from the Camerate type. Indeed, so far as I am aware, Jaekel (10) is the only writer oi* repute that has endeavoured to find the ancestor of Uintacrinus in this order. The Khodocrinidae, however, which are the forms he fixes on, are far removed from Uintacrinus, in the possession of branched, biserial arms, and interradials resting on the basals. This leaves, among Palaeozoic crinoids, from which one presumes that Mesozoic crinoids are descended, the Inadunata and the Ichthyocrinacea (=Articulata, W . & Sp., Flexibilia, Zittel). Most if not all of those palaeontologists that have attempted an answer have decided in favour of the Ichthyocrinacea, usually pitching on Forbesiocrinus as their example of a similar form. Held, as it has been, by Zittel (5 and 11), Neumayr (6), Carpenter (7), |
