OCR Text |
Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 1001 much the same thing occurs in the Pentacrinidae and Apiocrinidae. If Dadocrinus, or an allied form, is the ancestor of Pentacrinus in one direction, of Apiocrinus in another direction-then why not of Uintacrinus in a third direction ? As to the loss of the stem, it may have been by a process of gradual diminution, or in consequence of a habit of sudden separation. In favour of the former view are the instances of many species of Pentacrinus, which are known to move from place to place, and especially of Millericrinus pratti, the stem of which gradually dwindles to nothing. In favour of the latter view is the suggestive fact that in the slabs of fossil Dadocrinus the crown is almost always broken off from the stem either at, or immediately below, the junction of the stem with the cup ; and this must have taken place as a reflex response to some stimulus rather than as the result of force, for the crowns are otherwise undisturbed, and are still in natural juxtaposition with outer, then on the inner side, and so on regularly. One variation shows syzygies between secundibrachs 1 & 2, 3 & 4, 5 & 6. Another shows syzygies between secundibrachs 1 & 2, 3 & 4, 5 & 6, 8 & 9. Professor von Koenen has obliged Pig. 13. Badocrinus gracilis, portion of a specimen from Sacrau, near Gogolin iu Silesia, Brit. Mus. E 6070. The drawing shows basals, radials, primibrachs, secundibrachs (or distichals), pinnules, and interbrachials. Magnified 5 diameters. me by examining his numerous specimens with reference to this point, and admits that such differences from bis own account do occur. He is at variance with m e only in considering them "anomalous;" but there is nothing really anomalous about them, for the law of alternate pinnulation is never abrogated. The only question is, which type is the most usual. The one Von Koenen finds most often, I find most seldom. Both, however, agree with Uintacrinus so far as they go. |