34 TH. MORTENSEN, (Schwed. Sudpolar-Exp. 



anal system as usual (one of the plates is abnormally divided into three smaller plates; 

 not shown in the text figure); the anal plates form a rather conspicuous pyramid, are 

 not flat as in Dufresnii. The whole apical system finely striated granulated as in 

 Dufresnii. — The apical system of the specimen in hand affords the curious, evidently 

 abnormal feature that the madreporic pores have spread over the adjoining ocular 

 plate II and genital plate 2 (Fig. 9). 



The peristomial membrane encloses rather numerous, small, smooth plates; a few 

 larger plates outside the buccal plates carry numerous pedicellariae like the buccal 

 plates. — The gill-plate is short, not longer than wide, and not pointed distally. 



The spines are thicker and coarser than in Dufresnii; the cap generally has 

 three or more distinct longitudinal ridges, while in Dufresnii there are generally 

 only two (PI. XV Figs. 4, 8). I would, however, doubt that a constant specific 



Figs. 8—9. Apical system of Arbacia Dufresnii (8) and A. crassispina (9). 



difference could be found herein. In sections of the spines a slight difference is 

 likewise seen between this species and Dufresnii; the solid wedges in the periphery 

 having completely disappeared, whereas they are generally distinct in Dufresnii 

 (Figs. 10— II). The secondary abactinal spines are short and thick, clubshaped. — 

 The close covering of spines makes the species look very different from Dufresnii. 



The pedicellarias are, upon the whole, very like those of Dufresnii. The ophi- 

 cephalous pedicellariae occur in two forms (PI. XV Figs. 5, 7) as in Dufresnii and 

 afford only unimportant differences from those in that species; the stalk is alike. 

 The tridentate pedicellariae (PI. XV Fig. 14) perhaps a little less elongate than in 

 Dufresnii. No triphyllous pedicellariae were found. 



Spicules appear to be entirely wanting in the tubefeet in the specimen in hand. 



The colour of the test (abactinal side) is dark green, the tubercles white. The 

 spines of the single, dried specimen in hand are gray, those on the actinal side with 

 a slight reddish tint towards the tip and a faint greenish tint at the base. 



