236 



ANNULOIDA. 



of which disappear before reaching the poles. Each inter- 

 ambulacral plate carries a primary tubercle, not surrounded 

 by a ring. The ambulacral areas are composed of two rows 

 of plates. 



In the genera Melonites and Oligoporus, of the Carboniferous 

 rocks, we have large spherical Urchins, in which the test 

 appears to have been rigid, though some of the plates are 

 occasionally bevelled off, so as to articulate in an over- 

 lapping manner with one another. In Melonites (fig. 131) 

 there is a multiplication of the plates of both the interam- 

 bulacral and ambulacral areas, the former consisting in the 

 middle of about nine rows, while the latter are of eight rows, 



Fig. 131. A, Portion of an ambulacral area of Melonites multiporus. B, Portion of an 

 ambulacral area of Oligoporus Dance : i, Lateral row of interambulacral plates. Carbon- 

 iferous. (Meek and Worthen). 



or, in a British species, of from twelve to fourteen rows. 

 The central two rows of ambulacral plates are larger than 

 the rest and elevated above them, and each plate of these 

 areas is doubly perforated. The apical disc (fig. 130, B) is 

 composed of the normal ten plates, but the ocular plates 

 are sometimes imperf orate, and the genital plates are fur- 

 nished with from three to five pores. 



