187 
outline of the peristome is perfectly round; in specimens of ca. 10 
mm length it is distinctly transversely oval, while in the adult 
specimens it assumes a more pentagonal shape, the primary plate 
of the posterior interambulacrum being somewhat larger than the 
b. 
Fig. 20 . 
Fig 20. Apical system and posterior interradius of Echinobrissus recens ; 
a from a specimen 5 mm long, with the periproct still directly in contact 
with the apical system ; b. from a specimen 7 mm long ; the periproct nearly 
separated Irom the apical system; c. from a specimen 11 mm long; the periproct 
widely separated from the apical system. 21 /i. 
Fig. 21. Apical systems of Echinobrissus recens. a. from a specimen 50 mm long; 
b. from a specimen 40 mm long. 8 /i. 
other primary plates, forming an incipient labrum. In the youngest 
specimens the whole underside of the test is flat, the peristome 
being perfectly flush with the test. Gradually it then becomes 
more and more sunken, till in the adult it lies quite deep, with 
vertical borders, the whole underside being somewhat sunk in the 
midline. 
