OCR Text |
Show 998 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, and apparently by Schlueter (4), who, however, cleverly shirked giving a definite reply, this opinion demands respectful consideration. The opinion will be more acceptable if Onychocrinus be substituted for Forbesiocrinus. Por in Onychocrinus one finds what does not occur in Forbesiocrinus, namely a differentiation of the arms into two main branches, wdth a tendency for the armlets to be reduced to the size and regularity of pinnules. This tendency is most obvious in the species for which B . B . Bowley l proposed two generic names in a single paper, viz., Aristocrinus, or Callaway erinus, concavus. Considering the extreme difficulty that the most acute palaeontologists have met with in distinguishing the genera Ichthyocrinus, Taxocrinus, Forbesiocrinus, Onychocrinus, and their allies, considering the impossibility of deciding such a question with the assistance of the comparatively few specimens or species in the British Museum, and considering the confused nature of the large and scattered literature, I would not, on the slight evidence offered by Mr. Bowley, venture to pass any criticism on his action other than that the name Aristocrinus, or " the best crinoid," is singularly inappropriate. Names and minor differences apart, w e find in this group of forms many species with small and disappearing infrabasals, with interbrachials forming a flexible union between the rays, with occasional interdistichals, with the proximal primibrachs and secundibrachs broad-backed thin plates very like those of Uintacrinus, wdth an axial canal differentiated in at least the more distal brachials, wdth a flexible tegmen, and wdth flexible arms and cup; and some species with a distichal and sub-pinnulate arm-structure, and with two primibrachs in each ray. In all these features, then, there is a noteworthy resemblance; but a closer inspection will reveal many important points of difference. The species to which reference has been made have an anal area distinct in the cup, such as there is no trace of in Uintacrinus. This, however, might well disappear in course of evolution, especially in a free-swimming form, just as it has disappeared in Encrinus and in Antedon, although undoubtedly present in the ancestors of those two genera. It is more important to notice that the interradially situate plates of the Ichthyocrinidae are all of them true interbrachial plates of the secondary system; they are none of them modified pinnules. Indeed the pinnules are in no case advanced to such a stage that they could coalesce as in Uintacrinus. The most one can say is, that in some species of Ichthyocrinus the brachials seem to have been united laterally. Again, there are no traces of syzygial union in the arms of the Ichthyocrinidae. Indeed the arms are so much less differentiated, even in Onychocrinus, than they are in Uintacrinus, that if one supposes any links between the two forms, one must suppose a very long chain of them. But of this chain, not one link is known. Therefore, though I admit the force of the 1 " Description of a new Genus and five new Species of Fossils from the Devonian and Sub-Carboniferous Rocks of Missouri," Amer Geol xvi D D 217- 223 (Oct. 1895). V |