1896. } GENUS OF FOSSIL ECHINODERMS. 1005 
test, as in the Plesiocidaroida. In the second case strength was 
obtained by the development of a plate in the centre of the apical 
system, as in the Salentide, 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 
ot Tiarechinus. As Jackson remarks, in all echinids after 
Bothriocidaris there are only two plates in the second row of 
interambulacral plates, except in Z%arechinus, 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 Tiarechinus and 
the Paleozoic 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. 1a, b, & c. Lysechinus incongruens, from the Trias of St. Cassian. The 
test seen respectively from above, from below, and from the side. 
x 4 diam. 
2a, b, & ce, The same in outline; diagrammatic. 
3a, b, & ce. Tiarechinus princeps, seen from the same aspects. xX 6 diam. 
(After Neumayr. ) 
‘4a, b, & c, The same in outline; diagrammatic. (After Lovén.) 
5. Diagram of an interambulacrum and genital plate of Tiarechinus. 
6. + of the same in Lysechinus. 
l BR. T. Jackson, ‘Studies of Palxechinoidea,” Bull. Geol. Soc, Amer. val. vii. 
1896, p. 248. 
