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Show 1896.] GENUS OF FOSSIL ECHINODERMS. 1005 test, as in tho Plesiocidaroida. In the second case strength was obtained by the development of a plate in tbe centre of the apical system, as. in the Saleniidae, which first appear in the St. Cassian beds. The last point it is necessary to consider is from what possible ancestor the Plesiocidaroida may have been derived. I am not aware that any suggestion has ever been made as to the ancestry of Tiarechinus. As Jackson remarks, in all echinids after Bothriocldarls there are only two plates in the second row of interambulacral plates, except in Tiarechinus, where there are three, an arrangement which is " therefore to be looked at as a feature standing quite by itself as a structural detail" \ Lysechinus, however, bridges the gap in this respect between Tlarechinus and the Palaeozoic echinids. All those typical genera of the latter, in which none of the interambulacral plates pass on to the peristomal membrane, have the oral ends of the interambulacra arranged as in Lysechinus. In them a single peristomal plate is succeeded by two plates, above which occurs a line of three. Lysechinus is therefore the more primitive genus. The interambulacra of Tiarechinus can easily have been produced from it by the resorption of the second zone of interambulacral plates and increase in height of those of the third zone, so that they are left directly superposed on the single peristomal plate. The St. Cassian fauna is rich in new types of structure, which probably arose from the somewhat wild attempts of its members to adapt themselves to unfavourable conditions of life. Hence it appears more reasonable to regard the Plesiocidaroida as a random offshoot rather than as an ancestral group, and as being of interest as a biological backwater out of the main stream of echinid development, instead of being its primary source. EXPLANATION OF PLATE LI. Fig. la, b, & c. Lysechinus incongruens, from tbe Trias of St. Cassian. The test seen respectively from above, from below, and from the side. X 4 diani. 2 a, b, & c. The same in outline ; diagrammatic. 3 a, b, & c. Tiarechinus princeps, seen from the same aspects. X 6 diam. (After Neumayr.) 4a, b, & c. Tbe same in outline; diagrammatic. (After Loven.) 5. Diagram of an interambulacrum and genital plate of Tiarechinus. 6. „ of the same in Lysechinus. ] E. T. Jackson, " Studies of Paheechinoiden," Bull. Geol. Soc. Amer. vol. vii. 1896, p. 243. |
