62 



TH. MORTENSEN, 



(Schwed. Sudpolar-Exp. 



the periproctalregion of PL hirsntus (Fig. 14), which has not been given in the 

 ilngolf) Echinoidea, where this species was described.) The periproct is not sunken; 

 the whole posterior end sloping rather gently. — Regarding the other interambulacra 

 it is only to be noticed that the anterior paired ones agree with those of the other 

 species in the second plate being compound (2 + 2). — The anterior ambulacrum 

 is distinctly depressed towards the mouth; towards the apical system the plates 

 become somewhat lower than in the other ambulacra. In the posterior paired 

 ambulacra it is the 4th plate which reaches within the fascicle, in the usual way. 



Fig. 13. Fig. 14. 



Figs. 13—14. Periproctal region of Plixichinus Nordcnskjoldi (13) and PL hirsutus (14). '/i. 



The apical system, which is somewhat sunken along the median line, is very 

 interesting, being of a quite primitive structure (PI. XVII Fig. i). All the four 

 genital plates are distinct, each with a genital pore covered by a long genital pa- 

 pilla; they join in the middle, the anterior and posterior pair being separated only 

 by the anterior paired oculars, which likewise join in the middle. The apical 

 system thus affords a very conspicuous difference from the two other species of 

 the genus. 



The primary tubercles are more scarce than in PL liirsittus and cinctiis. On 

 the abactinal side most of the plates carry only one primary tubercle each, while 

 on the sides and on the actinal side there are 2 — 3, sometimes 4. primary tubercles 

 on each plate. For the rest the plates are closely covered with miliary tubercles. 

 Only few tubercles occur round the peristome, so that one cannot speak of a bourrelet 

 here. The ambulacral plates along the sternum are likewise covered with secondary 



