8 TH. MORTENSEN, (Schwed. Südpolar-Exp. 
The several small specimens enable me to give some information on the changes 
during growth of this species. 
In a specimen of 3.5 mm. diameter the test is somewhat pentagonal. The num- 
ber of coronal plates is - — as given in the measurements above — 6—7 ambulacral 
and 4 — 5 interambulacral plates. On the ambulacral plates only the primary tubercle 
has appeared as yet; the pores are small, and the two pores of each pair stand one 
above the other, almost vertically, a feature characteristic of young Cidarids, as 
pointed out by DöDERLEIN (Japanischen Seeigel. I. p. 30). On the peristome only 
two ambulacral plates are found in each series, the inner one the larger; no inter- 
ambulacral plates have appeared on the peristome, but the primary interambulacral 
plate is already almost wholly resorbed. The areoles are all confluent, not separated 
by secondary tubercles, which are represented only by a single series along the inner 
and outer side of the areoles. — The genital plates are comparatively very large, 
widely separating the small ocular plates from the likewise small anal system; the 
latter with a double circle of small plates. Only 3 pores in the madreporite. The 
genital plates carry rather many tubercles, the ocular plates only one each. The 
radioles are strongly thorny (PI. XIII P'ig. 6); these thorns, however, are not the 
same as those of the fullgrown radioles, no ostracum having appeared as yet (comp. 
Echinoiden der deutschen Südpol. -Exp. p. 9). The secondary spines are somewhat 
more thorny than in the grown specimens, all clubshaped, though evidently the same 
ones which become flattened later on. — The pedicellariæ do not differ essentially 
from those of the grown specimens, only they are, of course, much smaller. 
In a specimen of 6 mm. diameter the ambulacral plates number 11 — 12 in each 
series, the interambulacral plates 6. On the ambulacral plates at the ambitus the 
first secondary tubercle has appeared, at the lower or upper corner of the plate. 
The pores are less vertical. 3 — 4 ambulacral plates have appeared on the peristome, 
the inner one no longer exceeding those outside in size; no interambulacral plates 
have as yet appeared on the peristome. The upper areoles are separated by a row 
of secondary tubercles, and some few secondary tubercles have appeared outside the 
circle round the areoles. The apical system is not essentially different from that of 
the specimen of 3 mm., only some more (io) madreporic pores are found. Genital 
pores, of course, not yet developed. Tubercles rather numerous on all the apical 
plates, also the oculars. Radioles and secondary spines of the same character as in 
the specimen of 3 mm., only the thorns less prominent on the larger radioles. 
In the next stage represented, 18.5 mm. horizontal diameter, the number of 
interambulacral plates is still only 6 — 7, the enlargement of the specimens thus being 
almost exclusively due to the increasing size of the plates formed at a much 
earlier stage. The ambulacral plates, on the contrary, have increased considerably 
in number, to 27 — 28; there is still only one secondary tubercle inside the primary 
