Echinoid Tests, Cidaroida. 
77 
which is rather suddenly changed to a concave curve, and this again into a con- 
vexity ending in a wall that rises almost vertically from the flat scrobicule. The effect 
is that of a basal terrace rounded oft' (fig. 138, elevation). 
There is no scrobicular ring, but the whole extra-scrobicular surface is crowded with 
tubercles and miliaries in which the following arrangement can be detected (fig. 138). 
The interradial tract contains two main meridional rows, each consisting of clearly 
mamelonate and perforate tubercles of two sizes — secondaries and tertiaries — 
which alternate in sequence, so that one secondary and one tertiary is on each 
interambulacral plate. Since the plates themselves alternate, a secondary of one 
column lies at the same horizontal level as a tertiary in the adjoining column. The 
distinction in size between these secondaries and tertiaries is subject to much Var¬ 
iation. But they can be identified by their grouping, for the space between a sec¬ 
ondary and the tertiary on the same plate is not so great as that between the 
same secondary and the tertiary on the next plate in the same column. The mammiform 
bosses of both secondaries and tertiaries are surrounded by narrow scrobicules, 
scarcely, if at all, sunk, but rendered clear by the close-set Ornament of the remaining 
space. This consists of miliaries among which are scattered quaternary tubercles, 
slightly mamelonate. None of these latter bear any definite relation to the primary 
scrobicules. In the adradial tract, however, as seen in b, there seems to be a reg¬ 
ulär row of miliaries or quaternaries along the margins of the scrobicules, while 
between them and the adradial margin each plate bears 3 or 4 rather larger (? tert¬ 
iary) tubercles (fig. 135). 
The adradial suture is constructed on the plan described under T. persiniilis, 
but there are only about 7 denticles to each interambulacral, which therefore 
corresponded to about 7 ambulacrals (fig. 136). In the holotype a portion of the transverse 
suture between the interambulacrals is seen at the aboral margin of a scrobicule; 
it slopes slightly downwards and inwards, and has a distinct median groove. 1 A 
similar sutural face, more bevelled, is seen in specimen b (figs. 136, 137). 
The plates are thin, especially at the edges of the primary scrobicules, but 
are strengthened at the sutures. 
The following are measurements in millimetres : 
a 
b 
Height of interambulacral. 
2.4 
3.15 
Width » » (estimated) . 
6.6 
8.0 
Transverse diameter of scrobicule 
4.4? 
5.0 
Diameter at base of boss. 
2.0 
2.6 
» of platform. 
1.0 
1.2 
» of mamelon. 
0.5 
0.7 
Width of interradial tract. 
3.3 
— 
Diameter of a secondary tubercle . . 
0.7 
— 
Thickness of plate near margin . . ca. 
0.5 
ca. 0.5 
Relations of the Species. — Although the high development of the extra- 
scrobicular Ornament suggests at first a Diademoid rather than a Cidaroid, still the 
nature of the adradial suture inclines one to keep the species in Triadocidaris in 
the absence of definite contrary evidence from the ambulacra. Further the resembl- 
For discussion of this structure, see under Miocidaris (p 88). 
