40 



spines; from the first beginning it is easy to see, if we have a young pedicellaria 

 or a spine before us, tiie pedicellariæ always commencing with a separate calcareous 

 plate for each valve and one for the stalk, the latter developing from the upper end 

 downwards'), whereas the spines always commence with a single calcareous plate, 

 developing from the base towards the tip. I must decidedly maintain that the 

 pedicellariæ of E c h i n i d s are not transformed spines, they are 

 organs sui generis. The fact that some forms of pedicellariæ in Asterids are 

 only transformed spines does not alter the fact of the Echinid-pedicellariæ being 

 organs sui generis, as it may well be supposed that these organs have appeared 

 independently in the two classes, being thus, strictly speaking, not homologous 

 but only analogous structures. (Comp. Cuénot. Études morphol. sur les Echino- 

 dernies. p. 3ß8). 



Classification of the Diadematids. 



The family DiV/f/pmrt/irf«' was established by Peters '-) for the genera D/rtrfema, 

 Astropiiga, Echiiwthri.v and Centrostephanus, the two former genera, Diadema and 

 Astropyya having previously been referred to the Cidaridce (Gray) or to the Ecbi- 

 nidœ (by Agassiz & Desor); the two latter genera, Ecbinothrix and Centrostephanus 

 were established by Peters in the work quoted. Agassiz in his „Revision of 

 Echini" a.ssumed the family Diadematidœ in accordance with Peters, adding only 

 the genus Asthenosoma. Later on he has added to that family the genera Aspido- 

 dicidenm, Dermatodiadcnia and Micropyga, whereas Asthenosoma is transferred to the 

 family Echinothuridw. The genus Cœnopedina (Hemipedinu) Agassiz refers, as well 

 known, to the „Triplechinidœ'', though he otherwise regards it as being a subgenus 

 of Pseudodiadema. Pomel (Op. cit.) adopts the same arrangement, only Cœnopedina 

 is referred to the Pedinidœ. An essentially dilTerent classification is given by Duncan 

 in his important „Revision of the Genera and great Groups of the Echinoidea"''). 

 The genus Aspidodiadema is made the type of a distinct family Aspidodiadematidœ, 

 and the family Diadematidœ is divided into four subfamilies, viz. I. Subfam. Diadc- 

 matinœ, with the genus Uiadema, under which Centrostephanus, Hemipedina and 

 the fossil Microdiadema, Diademopsis and Ecliinodiadema are placed as subgenera, 

 and the fossil genera Placodiadema, Heterodiadema, Codiopsis, Pleurodiadema, Mag- 

 nosia and Cottaldia. II. Subfam. Diplopodiinœ, with the genus Micropi/ya and the 

 fossil genera Diplopodia, Pedinopsis, Acantheciiinus, Phijmeclùnus, Asteropsis, Diplo- 

 tuyma and Plistophijma. III. Subfam. Pedininœ , with the genera Echinothrix and 

 Astropyga and the fossil genera Pedina (with the subgenus Pseudopedina), Echino- 



') Ingolf-Ecliinoidea. I. p. 6. PI. XII. fig. 30 etc. 



') Über die an der Küste von Mossamliiquc l>e()l)aclitetcn Seeigel und in.sbesondcie üljcr die 

 Gruppe der Diademen. Abli. d. Berl.Akad. 1853. 

 ») .louni. Linii. Soc. Zoology. XXIII. 1891. 



