THE GENUS COLOBOCENTROTUS. 5 
plate, and the greatest width of the poriferous field is at the second and third 
plate from the actinostome, leaving a comparatively narrow interambulacral field 
(Pl. 12, fig. 1). From the ambitus to the abactinal system the large tubercles 
Fic. a. P. pedifera. Fic. b. P. atrata. 
occupy the whole of the ambulacral plate, leaving room only for a thin line of 
small miliaries. The abactinal tubercles alone have a circular scrobicular 
area; from the fourth plate from the apex it becomes irregular in outline, 
s 
Fic. c. Colob. Mertensii. Fic. d. Colob. Stimpsoni. 
and the pairs of pores are sometimes pushed in two arcs, or forced out towards 
the outer edge of the ambulacral plate. At the ambitus and on the actinal 
side the primary tubercles form two short rows on each side of the primary rows 
and then extend towards the actinostome as a single row of small secondaries 
