ECHINOIDEA. 



235 



lapping. The ambulacral areas are comparatively narrow, of 

 two rows only, each plate perforated by two pores. The in- 

 terambulacral areas are broad, and are composed of from five 



Fig. 129. Palaxhinus ellipticiis. The left-hand figure shows a portion of an ambulacral 

 area enlarged. The right-hand figure exhibits a single interambulacral plate. 



to eight rows of plates. The apical 'disc (fig. 130, A) has 

 five triply-perforated genital plates, and an equal number 

 of doubly-perforated ocular plates (Baily), but the latter are 

 not recognised by De Koninck. The genus is said to occur 

 in the Silurian, but is principally known as occurring in the 

 Carboniferous rocks. 



Fig. 130. A, Apical disc of Palcechimis, enlarged (after Baily). B, Apical disc of Melonites 

 (after Meek and Worthen) : g, One of the genital plates ; o, One of the ocular plates. 



The genus Eocidaris, of the Devonian, Carboniferous, and 

 Permian formations, also appears to have rigidly articulated 

 plates. The interambulacra are of five or more rows, some 



