Bd. VI: 4) 
THE ECHINOIDEA. 
27 
toujours déprimé» accordingly does not always hold good; the fact that all DE 
Loriol’s specimens came from a single locality may account for their uniformity. The 
outline of the test at the ambitus is generally angular, the ambulacra forming the 
prominent edges; sometimes, however, the interambulacra form the edges, and other 
specimens may be quite round. 
Dia- 
meter. 
Height. 
Apical 
system. 
Anal 
system. 
Peri- 
stome. 
Width of 
Number of plates. 
Longest 
spines. 
Ambulacra. 
Inter- 
ambulacra. 
Ambulacra. 
Inter- 
ambulacra. 
mm. 
mm. 
mm. 
mm. 
mm. 
mm. 
mm. 
mm. 
5 ° 
28 
13 
7-6 
24 
8 
23 
21 — 22 
V 
3 ° 
48 
26 
13 
7 ~ 6 . 5 
24 
8 
22 
vO 
1 
Ni 
O 
15 — 16 
22 
41 
30 
1 1 
6.5— 5-8 
18 
7 
19 
22-23 
17—18 
25 
37 
21 
1 1 
5-5 4- 5 
20 
7 
l6 
19—20 
15 — 16 
? 
37 * 
16 
10 
5 - 4-5 
20 
5-5 
16 
19—20 
14-15 
? 
31 
16 
8, s 
4-3 3 - 2 
16 
5 
13-5 
18—19 
14 
16 
29 
V 
8.5 
4 - 3-5 
15 
5 
12.5 
18—19 
14-15 
15 
18 
8.5 
6 5 
3 - 2-5 
10 
3-5 
7-5 
12—13 
IO — 1 1 
? ■ 
18 
8 
6. 5 
2.6— 2.2 
10 
3-2 
8 
V 
10 — II 
? 
H 
7 
6 
2.8 — 2.2 
8 
2-5 
6 
12 
10 
? 
The tuberculation is upon the whole fairly constant. All the specimens have a 
broad bare interambulacral space on the abactinal side. Secondary tubercles are 
scarcely found on the abactinal side, at most a small one near the primary tubercle 
on 2 — 4 plates above the ambitus. — The development of the epistroma round the 
primary interambulacral tubercles and in the ambulacra is rather variable, upon the 
whole not very strong (PI. XV Fig. 3). 
The genital openings appear early; they are present in the youngest specimen 
in hand, 14 mm. diameter. They lie mostly nearer to the middle of the plate than 
to the apex; so close to the apex as in the specimen figured in the »Kassier» Echini, 
PI. I. Fig. 4, I have never seen them. In some few specimens I have found a double 
pore in one of the plates. — The ocular pore is generally not to be seen from 
above, a knob growing out and covering it. This knob is connected with the outer 
end of the plate across the pore, and a partition wall is formed which may con- 
tinue down into the ocular pore, which is thus divided into two more or less distant 
pores. The same peculiar structure — first described by LovÉN in Tetrapygus niger 
( Echinocidaris nigra ) (Études s. 1. Éch. p. 67) — is found in the other species of 
Arbacia, more or less developed. In the A. crassispina, described below, this fea- 
ture is especially prominent (PI. XV Figs. 9, 1 1 ). — Though generally the ocular 
Type-specimen of A. alternans (Troschel) 
