82 Triassic Echinoderms of Bakony. 
The peristomial structures are well preserved in a, and rather less so in b 
The margin has a faint convex curve, slightly excavate on the interradius. At the 
adoral end the meridional ridges of the inner surface abut on the interambulacral 
processes of the discontinuous perignathic girdle. The margin itself forms a slight 
thickening or ridge, continuous round the peristomial border of the interambulacrum, 
and presenting no trace of connection with any ambulacral processes. From this 
ridge arise the two interambulacral processes, one on each side of the interradius. 
Each process is subtriangular in plan, having at its adoral end a truncate apex 
merged in the marginal ridge, from which the processes rise gradually to their 
broader bases, at the same time diverging slightly. Between the processes is a tract 
continuous with the marginal ridge and therefore raised slightly above the general 
level of the inner surface of the interambulacrum. It is subtriangular in outline, 
having a truncate aboral apex separating the bases of the processes. Viewed from 
the adambital end of the fragment, the elevated base of each process is seen to 
arch over a slight excavation in the process itself. (See figs 142, 143, 147, 148). 
The following are measurements in millimetres : 
a GC d é 
——>>>_ ———— ee 
ai a2 a3 small large small large 
Meridional diameter of plate . 2.9 32 35 55 43 31 30 3.3? 3.7 
Transverse diameter of plate . 40 5.7 7.0 7.4 ? Deaton). Geo 7/0) 
Distance of centre of tubercle 
from adradialymaroine =) -ye 2) Sle(nzees 2 On oul 
Approximate dia-\ meridional 2.4 2.9 
2:0, 220 neo) eng 
24s 50:0 ie ? 
is) 
on 
- 
bo 
wo ote 
meter of scrobicule ) transverse 2.4 3.0 4.0 4.6 3.4 4.1 ? ? 
Approximate diameter of tub- 
EFClEs J ci Be ie ey Oe Oo ? 22 — — -— 
i i meridional ) i. ? Oe lst loyal, 
Diam. of mamelon). transyerse( -© 1st | S=LO\ apn tele vaisian ae komm 
Thickness of plate near margin — — 0.7 0.65 045 05 O58 ? 0.5 
Thickness on interradius . . 09 — O6 — — —- —- — — 
Thickness at tubercle . . . — — 18 — — —- —- — — 
In a: length of adradial margin of a column, comprising 3 well-developed plates, 
circa 8.7; of b column, circa 7.3; thickness at highest point of perignathic processes, 
1.9; distance of that point from peristomial border, 2.9. 
Relations of the Species. — In the indefinite and flush or feebly sunk 
scrobicule, and in the tendency of the miliaries to radiate, 7. immunita differs from 
other species of Triadocidaris and reminds one of the associated fossils referred to 
Miocidaris planus and Anaulocidaris testudo, It resembles the latter still further in 
the grooving and bevelling of the transverse sutures, although it is clear that the 
plates were not flexibly united as they appear to have been in that species. Other 
differences from Anaulocidaris are that in 7. immunita the miliaries are coarser 
and closer, the mamelons are larger and are extended vertically rather than trans- 
versely, and the adradial suture is denticulate. 
The flattened adoral region of the interambulacrum, with its depressed interrad- 
ial tract, the width of that tract, and the fine extrascrobicular ornament, all are 
features that remind one of the Diademina. On the other hand many characters of 
