310 ECHINODERMATA— ECHINOZOA phylum iv 



ambulacral differentiation. This group appears last geologically and has its 

 füllest expression at the present time, The Exocycloida with an eccentric 

 periproct is a homogeneous group. The structure of the lantern (keeled 

 teeth) with other characters affiliate the basal members with the Stirodonta. 

 Of this Order the Holectypina on the basis of the lantern structure, perignathic 

 girdle and ambulacral detail, make the nearest approach to the Stirodonta. 

 The Clypeastrina, by the characters of the lantern, perignathic girdle and 

 petaloid ambulacra composed of simple plates, are further removed from the 

 primitive than the Holectypina. The Spatangina, which have lost the lantern 

 in adults and have attained an extreme of differentiation in ambulacral 

 structure, bilaterality and an eccentric peristome, may well be considered the 

 most specialised group of the Exocycloida, and therefore the furthest removed 

 from the primitive. 



Up to this point each order is characterised by having two columns of 

 ambulacral plates, and either one or two columns of plates in each inter- 

 ambulacral area. The next step in structural differentiation is two columns 

 of ambulacral and three columns of interambulacral plates. This is the 

 character of the Plesiocidaroida which is further marked by an exceptionally 

 large apical disc which is a primitive feature. 



The next step structurally is types with two or more columns of simple 

 ambulacral plates and three or more columns of interambulacral plates, with 

 a small apical disk which is a progressive character. The Echinocystoida 

 with eight or more columns of interambulacral plates fall in this group. 

 This Order is incompletely known, especially as regards the apical disk and 

 the periproct, which last appears to be eccentric in an interambulacrum, 

 separating it radically from all of the Perischoechinoida. The Palaeodiscidae 

 is the more primitive family, with two columns of ambulacral plates ; the 

 l^chinocystidae is the more specialised family with four columns of ambulacral 

 plates. 



The Perischoechinoida include all remaining Echini ; primitive as regards 

 the lantern, they are specialised in the interambulacrum, frequently in the 

 ambulacrum, and also in having a small apical disk. The Archaeocidaridae 

 have two columns of ambulacral plates and in so far are primitive, but they 

 have from four to eight columns of interambulacral plates ; very large spines 

 (for the Paleozoic) ; ambulacral and non-ambulacral plates on the peristome 

 and much resorption of the base of the corona, specialised. *rhe Lepido- 

 centridae have also two columns of ambulacral plates. The family attains 

 many (5 to 14) columns of interambulacral plates, progressive ; ambulacral 

 plates only on the peristome and no resorption of the base of the corona, 

 primitive. The Palaeechinidae may have two columns of ambulacral plates 

 only, but typically more (up to twelve) with three to eleven columns of 

 interambulacral plates ; plates not imbricate, secondary spines only, ambu- 

 lacral with non-ambulacral plates on the peristome, slight resorption of the 

 base of the corona. The Lepidesthidae may have two columns of ambulacral 

 plates only, but typically more (up to sixteen or twenty) with three to 

 thirteen columns of interambulacral plates, plates imbricate, primary and 

 secondary spines, or the latter only, ambulacral plates alone on the peristome, 

 no resorption of the base of the corona as far as known. 



In almost all of the above Orders and families by the study of stages in 

 development, characters have been found in which the individual repeats the 



