ECHINOIDEA 



307 



adambulacral columns, smaller within. Eccentric primary spines and tubercles 

 with secondaries on dorsal adambulacral plates, and secondaries only on 

 interambulacral plates of dorsal median columns. This peculiar and specialised 

 genus is known best from the type species P. irregularis (Meek and Worthen). 

 Devonian ; Great Britain. Lower Carboniferous ; Europe, North America. 



N 



Fig. 434. 



Lepidesthes colletti White. Lower Carboniferous ; Montgomery County, Indiana, x 2^/2. 

 Madreporite and periproctal plates distinet (after Jackson). 



Meekechinus Jackson (Fig. 435). Twenty columns of plates in each 

 ambulacral area. Three columns of plates in each interambulacral area. 

 Plates of uniform size, imbricating strongly. Small central primary spines 

 and tubercles with secondary spines and tubercles on ambulacral and inter- 

 ambulacral plates. Teeth distally serrate, a unique character. This genus 

 with a Single species is one of the most specialised of known Echini, also it 

 is the geologically latest representative of its family. The twenty columns 

 of ambulacral plates occur only near the mid-zone, as further dorsally less 

 columns exist. This is the only Echinoid from the Paleozoic in which 

 pedicellariae have yet been found. Permian ; North America. 



