8 TH. MORTENSEN, (Schwed. Sudpolar-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 DODERLEIN (Japanischen Seeigel. L 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 Fig. 6); these thorns, however, are not the 

 same as those of the fullgrown radioles, no ostracum having appeared as yet (comp. 

 Echinoiden der deutschen Siidpol.-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 pedicellariae 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 n — 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 (10) 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 



