188 
In the youngest specimens the periproct is still in direct contact 
with the apical system and is hardly at all sunken. In a specimen, 
7 mm long, it is nearly separated from the apical system, the upper 
interambulacral plates nearly joining in the midline above it; at 
a length of 8 mm it is separated from the apical system by 
two pairs of interambulacral plates. In the youngest specimens 
(5 _ 7 mm) the five ocular plates and the four genital plates are 
distinet, the madreporite being still, at least in the main, con- 
fined to the right anterior genital plate. At a size of 1 1 mm the 
madreporite has occupied also the left anterior genital plate, these 
two plates being no longer distinetly limited against one another. 
The two posterior genital plates, on the other hånd, generally remain 
distinet also in the adult, the madreporite not encroaching upon them; 
sometimes, however, the madreporite occupies the whole of the 
apical system. (Figs. 20—21). The genital pores may appear at 
a size of ca. 9 mm; in a specimen 11 mm long they have, how¬ 
ever, not yet been formed. 
The petals begin to form rather early; at a size of 8 — 9 mm 
there are 3 — 4 petaloid pores in each series, at a size of 11 mm 
there are 8 of them in the three anterior, 11 —12 in the posterior 
petals. In the largest specimen the number of the pores is 32 in 
the three anterior, 36 in the two posterior petals. The pores are 
not very distant, slightly conjugated. The arrangement of the pores 
in double series round the peristome has begun already in the 
youngest specimens in hånd. In the young specimens the ambu- 
lacral plates are distinetly seen to be arranged in triads. In each 
compound plate the lowermost primary plate is the largest, the 
middle a small, demi-plate, the uppermost a narrow, but complete 
plate. The lowermost carries a large, primary tubercle. (Fig. 22 a). 
Also each interambulacral plate carries a distinet primary tubercle 
in the younger specimens. The secondary tubercles, however, soon 
inerease greatly in number, and as they reach the same size as 
the primary tubercles the series of primary tubercles, which make 
a very conspicuous feature in the young, are soon quite indiscermble. 
The tubercles are distinetly crenulate and perforate. They are sur- 
rounded by fairly deep areoles, being placed excentrically therein, 
viz. at the lower anterior edge. (This arrangement evidently must 
be an adaptation to the habit of burrowing in a coarse, gravelly 
