270 PALAEONTOLOGY 



(8) Perhaps the most easily observed change of all is 

 that in the interpoviferous areas (i.e., the middle strip of the 

 petals, between the pore-pairs). These pass through a 

 series of stages (a) " smooth " ; (b) " sutured," i.e. the 

 sutures of the plates are clearly marked; (c) " inflated," 

 i.e. instead of being gently concave, they rise in a double 

 convexity ; (d) " subdivided," i.e. the middle line between 

 these convexities becomes a distinct groove ; and 

 (e) " divided," i.e. this groove becomes very deep and 

 narrow. 



The main division in the classification of Echinoidea is 

 between those with radial and bilateral symmetry, or 

 Regularia and Irregularia. The great differences between 

 these two sub-classes may be tabulated thus : 



REGULARIA. IRREGULARIA. 



Radial symmetry, perfect ex- Bilateral symmetry, perfect ex- 

 cept for madreporite. cept for apical system. 

 Anus within apical system, Anus in posterior interamb. 

 which is symmetrical and Apical system usually asym- 

 large. metric ; small and compact, 



or elongated. 



Mouth central, with well-de- Mouth either central (when jaws 

 veloped jaws. may be well- developed or re- 



duced), or in anterior amb 

 (when jaws are lost). 



Corona usually bearing large, Corona hardly ever with large, 

 prominent tubercles (as well conspicuous tubercles. Radi 

 as smaller ones) with long oles small, 

 stout radicles. 



The Palaeozoic echinoids are distinguished from all 

 later forms (except one Triassic and one Barremian 

 genus) by the inconstancy in the number of coronal 

 columns of plates, which may be more or less than 

 twenty, but never that exact number, which is the in- 

 variable number among later forms (with the one excep- 



