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Show 1004 DR. J. W. GREGORY ON A NEW [Dec. 15, the crinoids was then in the ascendency. The strikingly crinoid-aspect of the dorsal half of tbe test of Tlarechinus was held to support this theory by showing that the apical plates were of great functional importance in the primitive echinids. The same line of argument would tend to connect Lysechinus with the Stellerida ; for the ambulacra are " lysactinic," or limited to grooves on the oral surface, and the dorsal surface is somewhat like that of such an Ophiurid as Ophlopyrgus 1. But in spite of the temptation to deduce the characters and affinities of the primitive echinid from these two genera, I am bound to confess that they appear to give no information whatever upon this subject. In the first place they came too late to be ancestral; they may be primitive, but they are not primaeval. The Echinoidea began in the Ordovician. The Plesiocidaroida do not appear till the Trias. It is idle therefore to regard the Triassic caliculate Tlarechinus as the ancestor of the Silurian acaliculate Echinocgstis. The Plesiocidaroida resemble the Mesozoic genera Salenia and Acrosalcnla in the size of the apical area, and Cldarls in the arrangement of the ambulacral plates, rather than any of the Palaeozoic families such as the Archaeocidariidae, Melonitidae, or Palaeechinidae. When the order is compared with its predecessors its characters appear specialized instead of primitive, and it appears more reasonable to regard it as an aberrant offshoot from some Palaeozoic echinid, rather than a close relation of the ancestor of the class. This idea is quite in harmony with the evidence as to the physical conditions under which the members of the two genera lived. They both come from the Trias near St. Cassian. Lysechinus probably came from the neighbourhood of Sett Sass, and from the Middle St. Cassian or " Stuores zone." The rock-sequence of the Trias in this area 2 includes a variable series of volcanic tuffs, grits, and agglomerates, massive and nodular drusy dolomites, coral-reefs, and thin-bedded limestones. The sequence indicates considerable volcanic disturbances and very variable conditions ; lagoons, no doubt, occurred among the coral-reefs, and if these became saline the animals in them would be stunted in development. Animal life was prolific in this warm sea, but the conditions were unfavourable to normal development. Hence the fossils-corals, sponges, echinids, and mollusca-are all small and stunted. The animals appear to have dwindled in size as the conditions became more and more adverse. As the echinids became smaller the tests appear to have needed strengthening, which was managed in two different ways. In the first the apical plates increased until they covered the whole upper half of the 1 Th. Lyman, " Eeport on tbe Ophiuroidea," Eep. Chall. Exped., Zool. vol. v. 1882, p. 33, pl, ix. figs. 16, 17. 2 See e.g. M . M . Ogilvie, " Contributions to the Geology of the Wengen and St. Cassian Strata in Southern Tyrol," Quart. Journ. Geol. Soc. vol. xfix. 1893, p. 22, and table facing p. 16. |
