76 ECHINODERMATA. ECHINOIDEA. 



sea-urchins the rows of pores extend from the apical 

 disc to the mouth without any important alteration, the 

 ambulacral areas are then said to be simple (e.g. Cidaris}. 

 In others the rows of pores diverge soon after leaving 

 the apical disc, and then come together again, so as to 

 form a rosette on the upper surface of the corona, the 

 pores only being irregularly developed on the lower, the 

 ambulacral areas are then termed petaloid (e.g. Scutella), 

 and when the rows of pores diverge but do not come in 

 contact at their lower ends they are sub-petaloid (e.g. 

 Echinobrissus). The plates of both the ambulacral and 

 interambulacral areas are often provided with rounded 

 elevations known as tubercles and granules. The tubercles 

 are of various sizes, the largest being the primary. In 



FIG. 20. A. Spine of Cidaris florigemma, from the Corallian Rocks, 

 a, acetabulum ; h, head ; c, collar ; 6, body or stem. B. Ambu- 

 lacral plate of Cidaris (recent) with a large primary tubercle and 

 secondary tubercles. In the primary tubercle, m, mamelon ; b, boss ; 

 , areola. 



these the following parts may be distinguished ; at the 

 summit there is a spheroidal piece, which is sometimes 

 perforated, this is the mamelon (fig. 20 B, m ). The 

 mamelon rests on a short pillar termed the boss (b), the 



