22 NATURAL FAMILIES. 



test is extremely thin, and is covered with small tubercles, which support hair-like spines. 

 Besides these, there are some larger crenulated and perforated tubercles, which support 

 large spines. There are two or four genital pores, which are sometimes placed close 

 together, but are in other genera apart. The eye-plates are five in number, and are placed 

 at the apices of the ambulacra, in a pentagonal form, around the genital plates. On the 

 surface of the test of some Spatanc/ida certain delicate lines called fascioles are observed, 

 having a smoother appearance than the tubercular surface of the test : they are furrows, 

 strewed with microscopic tubercles, and destined to carry very delicate spines, which, 

 when seen under the microscope, appear to have the same structure as the PedicellaricB. 

 The fascioles have a different disposition in each genus, and afford a good character in 

 making definitions of the same. When the fasciole surrounds the ambulacral petals like an 

 undulating groove, as in Hemiaster, Schizaster, &c., it is said to he peri-pefaious j when it 

 surrounds the single ambulacrum, as in Anip/iidetus, it is internal ; when it extends along 

 the sides, as in ScJdzaster, 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 the sub- 

 anal and peri-petal are frequently associated together. 



Types. Spatangus purpureus, Miiller. Brisms lyrifer, Forbes. Brissopsis Ducieii, 

 Wright. 



