1004 Dn. J. w. qregout os a new [Dec. ISj 



the erinoids was then in the ascendency. The strikingly crinoid- 

 aspect of the dorsal half of the test of Tiareehinus 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 Lysecldrms 

 witli 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 Opliiopyrgxis '. 



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 priniiBval. The 

 Ecbinoidea began in the Ordovician. The Plesiocidaroida do not 

 appear till the Trias. It is idle therefore to regard the Triassic 

 caliculate Tiareehinus as the ancestor of the Silurian acaliculate 

 Echinocystis. The Plesiocidaroida resemble the Mesozoic genera 

 Salenia and Acrosalenia in the size of the apical area, and Cidaris 

 in the arrangement of the ambulacral plates, rather than any of 

 the PaliEOzoic families such as the Archfeocidariidoe, Melonitidae, 

 or Palseechinidae. When the order is compared with its prede- 

 cessors its characters appear specialized instead of primitive, and it 

 appears more reasonable to regard it as an aberrant offshoot from 

 some Palajozoic 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. Lys- 

 echinus probably came from the neighbourhood of Sett Bass, and 

 from the Middle St.. Cassian or " Stuores zone." The rock- 

 sequence of the Trias in this area ' 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 



* Th. Lyman, " Eeport on the Ophiuroidea," Eep. Chall. Exped., Zool. vol. v. 

 1882, p. 33, pi. ix. flea. 16, 17. 



' See e. g. M. M. Ogilvie, " Contributions to the Geology of the Wongen and 

 St. Oassinn Strata in Soulliern Tyrol," Quart. Journ. Goal. Soc. vol. xlix. 1893, 

 p. 22, and table facing p. Ki. 



