96 
Triassic Echinoderms of ßa,kony. 
of the plate; here, as also in c, e, f, h, j, and k, the meridional diameter is rather 
less. The Perforation, however, tends to be drawn out meridionally, not horizontally. 
The surface of the mamelon slopes adorally. The neck is not undercut, but rests 
on a slight platform, vvhich may surround it (as in j), or may be absent on the 
adapical side (as in f and h), in which case the neck merges imperceptibly into 
the boss on that side. On the adoral side, however, the platform is always present, 
and, though it has no definite parapet, it is hollowed on this side. The boss is 
steeper on the adapical side, especially when it merges into the neck, and it has 
on this side a straighter slope; on the adoral side it slopes more gently, with a 
convex curve, often with a slight median depression continuing the adoral excavation 
of the platform. It passes into a smooth area, which may be very slightly depressed, 
but is usually flush, or even (as in h) raised, reminding one in the last event of a basal 
terrace. This scrobicule merges quite gradually and indefinitely into the general sur¬ 
face of the plate. 
The extra-scrobicular surface bears miliaries, quite irregularly disposed and 
forming no definite scrobicular ring. These rise out of the plate in such a way 
as to appear to radiate from the primary tubercle. Sometimes they are obscurely 
mamelonate, and may be termed minute tubercles. Their variability of number 
and arrangement may be gathered from the figures (flgs. 164, 169, 170, 173, 175, 
178, 184, 187). They appear always to pass round the whole plate, so that the 
scrobicules, although indefinite, are never confiuent. 
Measurements in millimetres: 
a 
b c 
d 
e 
/ 
li 
j 
k 
Vertical diameter of plate . 
6.4 
4.6+ 5.7 
3.5 
3.7 
5.5 
6.9 
7.5 
5.9+ 
Transverse » » » 
7.5 
7.9+ 5.8+ 5.7 
4.6 
4- 
o 
+ 
7.7 
8.2 + 
8.1 + 
Distance of centre of tubercle 
from adambulacral margin . 
4.1 
3.6+ 3.5 
2.7 
3.0 
3.9 
4.2 
5.2 
3.3 
Approximate diameter of scrobi¬ 
cule . 
3.5 
3.7 2.5 
2.5 
2.5 
3.4 
3.9 
4.0 
4.6 
t transverse 
1.0 
1.0 
0.6 
0.7 
1.2 
1.4 
1.4 
1.2 
Diam. of mamelon j vertical 
0.85 
!- 15 0.5 
0.6 
0.65 
0.8 
0.9 
0.9 
1.0 
Thickness of plate near margin 
0.6 
0.7 0.4 
0.4 
0.5 
0.5 
0.6 
0.6 
0.6 
Relations of these’ Interambulacrals to the Radioles described as 
Anaulocidaris testudo. — In the absence of definite and close association of these two 
structures, the reasons for referring both to a single species may be stated as fol- 
lows : (1) The plates all come from that horizon and from those two localities which 
have furnished the largest number of characteristic Anaulocidaris radioles. (2) The 
tenuity of the plates and their loose union cannot be explained as due to abyssal 
conditions or to any deficiency of lime in the water; they suggest therefore that 
the animal needed some protection, such as, on our hypothesis, would have been 
afförded by the awning or «testudo» of radioles. (3) The absence of scrobicular 
tubercles and the reduction of all extra-scrobicular Ornament to miliaries or, as may 
sometimes have been the case, mere granules devoid of radioles, imply that the protection 
which in Cidaridae is universally supplied by the subsidiary radioles, was no longer 
needed; a elosely fitting «testudo» would, it is evident, deprive any such radioles 
of their utility. (4) The peculiar transverse extension of the mamelon, combined 
