THE ECHINOIDEA. 
Bd. VI: 4) 
63 
and a few primary tubercles. — a conspicuous difference from the smooth plates in 
the two other species. 
The primary spines are rather coarse, pointed, slightly curved, with none or 
only a few serrations; the spines of the actinal plastron are likewise pointed, not 
widened in the end; those around the peristome are not flattened. They all stand 
out very prominently, white against the more or less violet ground colour of the 
test. Those of the subanal plastron are the longest, 2.5 mm. in a specimen of g mm. 
length. — The secondary spines are widened at the point and somewhat curved 
(PI. XVIII Fig. 9). 
The tubefeet on the inner circle of plates round the peristome are penicillate, 
with only 1 — 4 thick filaments, supported by very strong, irregular, fenestrate rods 
(PI. XVIII Fig. 10). No spicules were found in these tubefeet. The simple tubefeet 
following next have a small cap of calcareous network in the point, and some few 
simple spicules occur in their walls. 
The pedicellariæ are rather scarce; only tridentate, ophicephalous and triphyllous 
pedicellariæ have been observed. They are upon the whole very similar to those 
of PI. hirsutus , the tridentate ones being only a little more irregular (PI. XVIII 
Figs. 6, ii); bivalve specimens occur. The triphyllous pedicellariæ (PI. XVIII Fig. 7) 
are like small tridentate. A form corresponding to the globiferous pedicellariæ of 
PI. hirsutus I have not found. The ophicephalous pedicellariæ (PI. XVIII Figs. 5, 12) 
are more developed than those of PI. hirsutus , the blade being somewhat broader; 
the upper end of the stalk cup-shaped as usual. 
Sphæridiæ occur only on the plates nearest the peristome, also in the anterior 
ambulacrum. They are short, round, with rather prominent ridges (PL XVIII P'ig. 8). 
The colour was brownish-violet in the one specimen, white with faint traces of 
violet pigment in the other. 
Two specimens were taken at Station 17, on the Shag Rock Bank (53 0 34' S. 
43 0 23' W. 160 m. — Sand, gravel; bottom temperature + 2.05). The measure- 
ments are: 
It may be especially noticed that also the smaller specimen has its genital papillae 
developed. It is thus probably a very small form. 
This very interesting species differs conspicuously from the two other species 
known of the genus Plexechinus , PI. cinctus and hirsutus , especially by its primitive 
apical system, by the fasciole comprising only 3 pairs of interambulacral plates and 
upon the whole by the structure of its epiproctal region. Perhaps these characters 
Length. 
12 mm. 
Breadth. 
Height. 
7 mm. 
6 » 
9 » 
