Echinoid Radioles , Cidaris trigona. 
221 
become so marked as to produce an imbrication directed distalwards. This imbri- 
cation may cover the vvhole flattened face, and in a few instances may also be 
developed to a less degree in the proximal region of the other faces. In the inost 
pronounced forms the constituent granules are entirely merged and lost in the 
imbricating ridges, but as the ridges pass over the margins of the flattened face 
they are continuous with the lines of granules on the sides. In the British Museum 
specimens, at any rate, the imbrication is most marked and rnost extensive in the 
smallest radioles, but appears to change into transverse rovvs of granules in the 
larger ones, tili, in the largest of all, the Ornament is merely irregulär granulation, 
except for the trace of imbrication at the proximal end of the flattened face. These 
facts suggests that the imbrication may, so far as this species is concerned, be 
more primitive than the granulation. A more minute account of it will therefore 
be given in Connection with the micro-structure of the shaft. 
Quenstedt seems to have found the granulation finer on the distal crown. 
This is not confirmed by the British Museum specimens, which show the granules 
there as generally less equal in size, and less regulär in distribution. Perhaps 
Quenstedt only meant to imply, what is certainly the case, that the end-crown is 
always granulate, never imbricate. 
Quenstedt also drew attention to the longitudinal grooves sometimes seen on 
the back of the shaft at its extreme proximal end. These may be due partly to 
vertical concrescence of granules, partly to weathering, partly to the action of boring 
parasites. 
The handle is separated from the shaft by the proximal imbricate ridge, or 
by a distinct encircling line of granules. Its surface is smooth, but in exception- 
ally well-preserved specimens a faint longitudinal striation can be detected. It 
slopes suddenly to the relatively small base, the axis of which does not coincide 
with the central axis of the shaft but is nearer to the back, and often also nearer to 
one of the sides; thus the slope of the handle is gentlest towards the flattened face. 
The collerette is separated from the handle by a slight Step or faint ridge; 
it is very low, and appears only as a shallow groove above the annulus. 
The annulus is gently rounded, smooth, and not prominent. No previous 
writer has been able to confirm Münster’s description of it as finely crenelate; 
perhaps he alluded to the fine striation mentioned above. 
From the annulus the base slopes in a concavo-convex curve to the slightly 
raised, smooth, marginal rim of the acetabulum. 
The following are measurements of selected specimens in millimetres: 
Greatest length . . 
6-0 
7-4 
8'7 
11-4 
12-5 
133 
14-0 
16-1 
16-6 
17-0 
18-2 
20-2 
» sagittal diam. . 
2-3 
2-5 
3‘9 
4'3 
41 
8-2 
5'2 
7‘1 
8-2 
6-3 
7‘6 
9‘4 
» transverse diam. 
3‘6 
4-0 
6'2 
5-8 
6-4 
9‘3 
7'8 
10*8 
10-5 
9-5 
11‘9 
12-3 
Length of flattened 
face. 
T7 
6-2 
6-8 
00 
CO 
10'5 
97 
1F8 
ca. 10-8 
11-4 
12-8 
13-5 
16-0 
From acetabulum to 
distal end of handle 
1.2 
? 
1-8 
2-0 
F8 
? 
2-2 
2-6 
2-5 
3-5 
3-2 
From acetabulum to 
top of collerette . 
0-3 
? 
0'8 
0-9 
? 
? 
1-2 
L2 
1-2 
F5 
0-9 
Diameter of annulus 
0'6 
? 
F15 
L3 
Fl 
? 
? 
2-6 
F9 
2-3 
2’8 
L8 
