1004 DR. J. W. GREGORY ON A NEW (Dee. 15, 
the crinoids was then in the ascendency. The strikingly crinoid- 
aspect of the dorsal half of the test of Tiarechinus 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 Ophiopyrgus ’. 
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 primeval. The 
Echinoidea began in the Ordovician. The Plesiocidaroida do not 
appear till the Trias. It is idle therefore to regard the Triassic 
caliculate Ziarechinus 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 Paleozoic families such as the Archeocidariide, Melonitide, 
or Paleechinide. 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 Palsozoic 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 Sass, 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 
1 Th. Lyman, “ Report on the Ophiuroidea,” Rep. Chall. Exped., Zool. vol. vy, 
1882, p. 33, pl, ix. figs. 16, 17. 
> 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. xlix. 1893, 
p- 22, and table facing p. 16. 
