GENERA AKD GROUPS OF THE ECHIlSrOIDEA. 25 



1. Suborder Streptosomata. — Test more or less flexible, with 



external and internal branchiae. Ambulacral plates alone 

 continued beyond the peristome to the stoma. 



2. Suborder Stereosomata. — Test rigid, with external bran- 



chiae and rudimentary or absent internal branchiae ; with 

 isolated buccal ambulacral plates. ' 



III. Order. — Euechinoidea with an actinal, central peristome, 

 and the periproct situated beyond the dorso-central system 

 in the posterior interradium ; with external hranchiae ; with a 

 pair of pores or only one pore to an ambulacral plate ; with 

 feeble jaws and vertical teeth, or without these structures ; 

 with a variably constructed perignathic girdle. Spheridia 

 present. Exocyclic, oligoporous, Ectobranchiata. 



HOLECTYPOIDA. 



IV. Order. — Euechinoidea with an actinal peristome, a peri- 

 proct situated beyimd the dorso-central system, in the pos- 

 terior interradium ; with external branchiae ; with tentacular 

 pores in the interradia, and more than a pair to an am- 

 bulacral plate ; tentacles heteropodous in arrangement ; 

 with more or less horizontal and rarely vertical teeth, 

 and with jaws situated superiorly to the disconnected 

 perignathic girdle. Spheridia present. Exocyclic, petalo- 

 branchiate, polyporous, G-nathostomes. 



CLTPEASTEOIDA. 



V. Order. — Euechinoidea with an actinal or a frontal peristome, 



and a periproct situated beyond the dorso-central system, 

 in the posterior interradium ; without external branchiae, 

 jaws, teeth, and perignathic girdle. Spheridia present. 

 Exocyclic, anectobranchiate, Nodostomata. 



SPATANG-OIDA. 



1. Suborder Cassiduloidea. 



2. Suborder Spatanggidea. 



