54 
TH. MORTENSEN, 
(Schwed. Südpolar-Exp. 
here may be two distinct series of tubercles between each two arcs of pores, the 
lower series remaining the largest. These tubercles may vary somewhat in size, but 
generally they are nearly so large as the inner secondary tubercle, and the oblique 
rows formed by them make a prominent feature of the species. — The pore-areas 
are scarcely narrowing towards the peristome, the ambulacra being here distinctly 
wider than the interambulacra. 
The primary interambulacral tubercles form a pair of prominent vertical series; 
mostly they are considerably larger than the primary ambulacral tubercles, but some- 
times they are not much larger than the latter; they scarcely diminish in size to- 
wards the apical system, whereas they diminish considerably in size towards the 
peristome. The secondary tubercles are not very prominent. Only in the largest 
specimen they are almost as large as the primary ones, forming at and just below 
the ambitus a distinct, regular vertical row inside and a more irregular vertical row 
outside the primary series (PI. VI. Fig. 6). In the other specimens they are con- 
siderably smaller than the primary tubercles. Each plate (at the ambitus) carries 
generally two larger secondary tubercles inside the primary tubercle off the upper 
edge of the latter, the median one being the larger, and a corresponding pair out- 
side the primary tubercle, the outer one generally being the larger. There is thus 
an indication of an arrangement in horizontal rows of these tubercles, while the larger 
inner tubercles form more or less distinct longitudinal series. — In the specimen 
figured in PI. VI. Figs. 1 — 2, 5 the secondary tubercles are perhaps somewhat ex- 
ceptionally small. The rest of the plates is closely covered by miliary tubercles, no 
naked median space being left. 
The apical system is held by Bell to be especially characteristic in having all 
the ocular plates excluded from the periproct. This seems also to be the general 
rule, but it is no constant character. Among the specimens examined by me, 1 1 in 
all, 3 specimens (among which the smallest) have 2 ocular plates in contact with 
the periproct viz. plates I and V in two specimens, IV and V in one. According 
to Bell (Observations on the characters of the Echinoidea. IV. The Echinometri- 
dæ. Proc. Zool. Soc. 1881, p. 429) even 4 ocular plates may be in contact. The 
genital plates have mostly a rather prominent series of tubercles along their inner 
edge. Sometimes two pores are found in one genital plate. The madreporic plate 
is very large, especially in the larger specimens. The anal opening is nearly central. 
The anal plates are rather large, carrying tubercles; there is no distinct central plate. 
The buccal membrane contains only few small plates outside the buccal plates, 
most of them rather thick, carrying a few triphyllous, sometimes also some ophi- 
cephalous pedicellariæ. Numerous small plates are found inside the buccal plates! 
the latter carry rather numerous pedicellariæ, ophicephalous and triphyllous, but no 
spines. 
