THE GENUS COLOBOCENTROTUS. 



plate, and the greatest width of the poriferous field is at the second and third 

 plate from the aetinostome, leaving a comparatively narrow interambulacral field 

 (PI. 12, fig. 1). From the ambitus to the abactinal system the large tubercles 



Fig. a. P. pedifera. 



Fig. b. P. atrata 



occupy the whole of the ambulacral plate, leaving room only for a thin line of 

 small miliaiies. The abactinal tubercles alone have a circular scrobicular 

 area ; from the fourth plate from the apex it becomes irregular in outline, 



Fig. 



Coloh. HJerlensii. 



Colob. Stiiiipsoni. 



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 aetinostome as a single row of small secondaries 



