Bd. VI: 4) 
THE ECHINOIDEA. 
33 
This change in size of the tubercles above the ambitus looks not so abrupt as in 
A. Dufresnii , on account of the occurrence of larger tubercles between the small 
ones. Near the apical system the two series become irregular, as is commonly the 
case in the species of Arbacia. The large tubercles at the ambitus occupy the 
whole height of the compound plate, the upper and lower edge of their base being 
more or less straight. The side of the tubercle-base looking towards the pores is 
somewhat deeply indented. The median line of the ambulacra is covered by nu- 
merous small miliary tubercles carrying pedicellariæ. The pores are arranged in a 
somewhat sinuating line. The epistroma is very strongly developed above and out- 
side each pair of pores (on the abactinal side) — a conspicuous difference from Du- 
fresnii , where it is very slightly developed (PI. XV Fig. i to compare with PL XV 
Fig. 3). Towards the peristome the pore area widens very considerably, being 
twice as broad as the interporiferous zone, and the whole ambulacral area is at the 
edge of the peristome twice as broad as the interambulacral area. 
The interambulacral plates carry at the ambitus each three large tubercles, some- 
times with a small fourth tubercle at the inner edge; there are thus 6 vertical series 
of interambulacral tubercles. The 2 — 3 plates immediately above the ambitus carry 
each four tubercles, gradually diminishing in size from the outer towards the inner 
one. The third of these tubercles continues until the 4 — 5th plate from above, the 
second continues up to the apical system, only on the uppermost plate it has not yet 
appeared. The outer series likewise continues to the apical system, without any inter- 
ruption; the tubercles gradually diminish in size, only exceptionally a single tubercle 
may occur, which exceeds the one immediately below somewhat in size. — The 
space between the primary tubercles is on the actinal side occupied by small miliary 
tubercles, carrying pedicellariæ; on the abactinal side a fairly well developed epi- 
stroma occupies the part of the plates not covered by the tubercles, together with 
the white miliary tubercles, carrying pedicellariæ, scattered between the elevations of 
the epistroma. — The inner edge of the plates is finely granular. — The strong 
development of the abactinal interambulacral tubercles (and spines), leaving scarcely 
any naked median space, forms a most conspicuous difference from Dufresnii, which 
latter species has always a very large naked space in the interambulacra on the ab- 
actinal side. (PI. V Figs. 2 — 3 to compare with PL V Figs. 4 — 7, 9 — 12.) 
The apical system upon the whole bears a considerable resemblance to that of 
Dufresnii; the shape of the genital plates is alike, and the ocular plates are all ex- 
cluded from the anal area as in that species (Figs. 8 — 9). The shape of the ocular 
plates, however, differs from that in Dufresnii ; the outer sides are longer and some- 
what concave, and the whole plate is conspicuously smaller than in Dufresnii; also 
the outer edge is somewhat different (PL XV Fig. 1 1 comp, with PL XV Fig. 9). 
The ocular pore is, as usually in Arbacia , covered by a prominent tubercle. The 
5 — 100133. Schwedische Südpolar-Expedition rçoz — içoj. 
