34 ECHINOIDEA. 



crenulated and perforated tubercles for supporting large spines. There are two or four 

 genital pores, placed close together in some genera, but apart in others. The eye- 

 plates, five in number, are situated at the apices of the ambulacra, in a pentagonal form, 

 around the genital plates. On the surface of the test of some Spatan(/id(B certain delicate 

 lines are observed, having a smoother appearance than the tubercular surface of the test ; 

 these are the /asc?o/(?s, which are strewed with microscopic tubercles, and destined to carry 

 very delicate spines. The fascicles have a different disposition in each genus, and afford a 

 good character in making definitions of the same. When the fasciole surrounds the 

 ambulateral petals like an undulating groove, as in Hcmiaster, Schizaster, &c., it is jjeri- 

 petalous ; when it surrounds the single ambulacrum, as in Amjjhidetus, it is internal; 

 when it extends along the sides, as in ScMzaMer, it is lateral ; when it surrounds the 

 circumference of the test, as in Pericosmus, it is marginal ; when it is limited to the base 

 of the anal opening, it is sub-anal. Sometimes, in the same genus, more fascioles than one 

 exist ; thus tlie sub-anal and peri-petalous are frequently associated together. 



Types. SjMtangus purpureas, Miiller. Brissu^ lyrifer, Forbes. Brissopsis Duciei, 

 Wright. 



