54 TH. MORTENSEN, (Schwed. Siidpolar-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- 

 das. Proc. Zool. Soc. 188 1, 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 pedicellaria.'. Numerous small plates are found inside the buccal plates? 

 the latter carry rather numerous pedicellariae, ophicephalous and triphyllous, but no 

 spines. 



