24 PEor. p. M. DUisrcAisr's eetisiok of the 



same species and specimen, and it and the opposite condition will 

 not- distinguish genera for the future *. 



The nature of the ambulacral plates, the arrangement of the 

 pairs of pores, and the shape and functions of the tentacles are, how- 

 ever, considered to be of great taxonomic importance. The great 

 importance of the radial plates, formerly erroneously considered 

 to have a reference to an optic organ instead of a primary tentacle, 

 is not admitted ; and M. Pomel is not followed when he endeavours 

 to establish genera and subgenera upon the entrance or not of 

 one or more of the radial plates within the periproctal ring. 

 Following A. Agassiz, the structure and shape of the spines are 

 considered of secondary importance in classification, and genera 

 are not separated on account of dissimilarities in the spines, 

 nearly or quite all other structures being the same. These 

 remarks will account for a proceeding which may take some 

 palaeontologists by surprise, namely, the absorption of Pseudo- 

 diadema into the previously diagnosed genus Diadema, and the 

 union with this last of many subgenera which were formerly 

 considered to be genera. 



II. Subclass IIUECIIINOIBEA. 



I. Order. — Euechinoidea with an actinal, central peristome and an 



abactinal periproct, situated within the dorso-central system ; 

 */ with internal, bran chise only, and having jaws and more or 

 less vertically placed teeth, and a discontinuous perignathic 

 girdle ; the interradial as well as ambulacral plates continued 

 beyond the peristome to the true mouth. Spheridia absent. 

 Endocyclic, anecto branchiate, Grnathostomes. 



^^ CIDAROIDA. 



II. Order. — Euechinoidea with an actinal, central peristome and 



an abactinal periproct situated within the dorso-central 

 system ; with perfect or rudimentary or absent internal 

 branchiae, with external branchiae and incisions in the peri- 

 stome ; with jaws and teeth and a continuous perignathic 

 girdle ; ambulacral plates alone continued beyond the peri- 

 stome or as separate buccal plates. Spheridia present. 

 Endocyclic, ectobranchiate, Grnathostomes. 



DIADEMATOIDA. 



* Ann. & Mag. Nat. Hist. 1888, pp. 120-122. 



