. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. ECHINOIDEA. II. 75. interambulacra (posterior series) is a little uncertain, as I was unable to see distinctly the limit betwen it and the ankylosed genital plate. There are only two genital openings, covered by long genital papillae; it is probably the an- terior pair which is found, the posterior pair having disappeared, evidently because there is no room for more than one pair of genital organs. The madre- poric pores are rather few in number (PI. VI. Fig. 17), placed behind the genital pores; in the specimen of 26™'"
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. ECHINOIDEA. II. 75. interambulacra (posterior series) is a little uncertain, as I was unable to see distinctly the limit betwen it and the ankylosed genital plate. There are only two genital openings, covered by long genital papillae; it is probably the an- terior pair which is found, the posterior pair having disappeared, evidently because there is no room for more than one pair of genital organs. The madre- poric pores are rather few in number (PI. VI. Fig. 17), placed behind the genital pores; in the specimen of 26™'" there are only two madreporic pores. The genital openings are present only in the two larger specimens and in a separated head-end. The smaller specimen shows no trace of genital openings. This species thus is not mature till a rather considerable size, since a specimen of 22mm is im- mature. The primary spines are rather scarce, only along the actinal and abactinal keel they are close-set; also along the anterior border they are more numerous; there is no serial arrangement of the spines. They are all short, the longest scarcely reaching 3,nm length; they are curved, widened towards the point, which is generally bifid (PI. XL Fig. 44); they are more or less serrate, generally more on one side than on the other. Those along the plastron are somewhat more widened than the abactinal ones; those on the posterior end of the abactinal keel bend down over the anal area. The spines within the oral invagination (PI. XI. Fig. 21) are, as usual, coarser and stronger than those on the outside; they are curved and more or less sharply serrate along the concave side. The miliary spines (PI. XI. Fig. 43) are likewise rather scarce in number; they are only ca. o-5'n,n in length, curved towards the point which forms a somewhat widened, slightly fenestrated plate. The clavulae of the fasciole are somewhat stronger, with a rather complicated widening at the point (PL XI. Fig. 42). The tube-f
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