CLASR I ECHINOIDEA 299 



Order 6. ECHINOCYSTOIDA Jackson. 



Test irregulär, periproct apparently in 'an interamhulacrum. Two to four 

 columns of plates in an ambulacral area, and eight to nine columns of plates in an 

 interamlmlacral area. Plates thin, imbricating. Oculars and genitals doubtful. 

 Silurian. 



Members of this order have been considered primitive, but structural 

 evidence is opposed to this view. This order includes the only exocyclic 

 Echini excepting that of the Exocycloida. The species are incompletely 

 known. 



Family 1. Palaeodiscidae Gregory. 



Two columns of plates in an ambulacral area and eight to nine columns of plates 

 in an interambulacral area. Primordial interambulacral plates in basicoronal row. 

 Base of corona not resorbed, Peristome with ambulacral plates only. Lantern 

 inclined, typically echinoid. Numerous fine spines. 



Palaeodiscus Salter. With characters of the family. The only genus 

 known, many rows of ambulacral plates on the peristome. Silurian ; 

 England. 



Family 2. Echinocystidae Gregory. 



Four columns of plates in an ambulacral area, and eight columns of plates in an 

 interambulacral area. Small primary spines and tubercles. Jaws. 



Echinocystites Wyv. Thomson (Cystocidaris Zittel). With characters of the 

 family. The only genus known. Silurian ; England. 



Order?. PERISCHOEOHINOIDA M'Coy. 



Test regulär, periproct within the oculogenital ring. Two to twenty columns of 

 simple plates in each ambulacral area, and three to fourteen columns of plates in each 

 interambulacral area. Plates imbricate or not. Primordial ambulacral plates on 

 peristome. Primordial interambulacral plates in basicoronal row, or resorbed. Base 

 of Corona not resorbed or resorbed. Oculars usually all insert. Genitals small, 

 typically with more than one pore each ; rarely (Lepidechinus) with one pore each. 

 Madreporite usually not recognisable. Periproct covered with many thick plates. 

 Peristome with many rows of ambulacral plates only, oi' in addition with interradial 

 non-ambulacral plates. Lantern inclined, composed of forty pieces, teeth grooved, 

 foramen magnum moderately deep, epiphyses narrow, no pits in top of pyramids. 

 Spines primary and secondary, or the latter only. Primary tubercles perfm'ate, 

 secondary tubercles imperforate. Silurian to Permian. 



This Order includes the majority of Paleozoic Echini. All are specialised 

 in having multiple columns of interambulacral plates, and in many genera 

 multiple columns of simple ambulacral plates as well ; the order is therefore 

 considered a further remove from the primitive than are those Orders 

 with two columns of both interambulacral and ambulacral plates. 



