Ö2 
TH. MORTENSEN, 
(Schwed. Südpolar-Exp. 
the periproctalregion of PL hirsutus (Fig. 14), which has not been given in the 
»Ingolf» 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 fasciole, in the usual way. 
The apical system, which is somewhat sunken along the median line, is very 
interesting, being of a quite primitive structure (PI. XVII Fig. 1). 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 PI. hirsutus and cine t us. 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 
