54 



Forms like Tetiacidaiis and Sperosoma also show this character to be less exclusive. 

 — It is interesting to note that in both Cidarids and Echinothurids we thus know 

 a form with pluriseriate plates. 



Jackson finds an important stage of development in the condition with only 

 one intcrambulacral plate and two ambulacral plates, as in Bothriocidaris; he 

 names it the „Protechinus"-.stage. It is found, indeed in all Echini. For the 

 Echinothurids it has not hitherto been indicated to exist. I can, however, assert 

 that it is reallj' found; it is seen very distinctly in young Phormosoma placenta. 

 I give here a figure of this stage (Fig. 4). 



It now remains to group the families of the Stereo- 

 soniata. The characters upon which such an arran- 

 gement must be undertaken are: the structure of the 

 ambulacra — diadematoid or echinoid — and the struc- 

 ture of the tubercles — perforate or imperforate — (the 

 crenulation is evidently of much less value). Finally 

 the structure of the teeth — keeled or unkeeled — is 

 evidently a very important character, though it has 

 hitherto received very little attention. Other more im- 

 portant characters I do not find; but when we group the 

 families according to these three characters an arrange- 

 ment results which seems to answer fairly well to the na- 

 tural relations of the different families. — Unfortunately 

 the structure of the ambulacra and , especially, of the 

 teeth is unknown in a great number of the fossil forms. 

 But it is likely that these structures will be known by and by, the teeth being 

 not so very seldom preserved in the fossil Echini. For the present we must mainly 

 make our conclusions from the recent forms. 



An examination of the different families with regard to these features then 

 gives the following result. Diadematoid ambulacra, perforate tubercles and un- 

 keeled teeth are found in the fam. Diadematida', Micropygidæ and Pedinidœ^): the 

 Aspidodiadematidce certainly belong to the same group, their simple ambulacra 

 evidently representing only a primary stage of the diadematoid ambulacra (in 

 Aspidodiadema the ambulacral plates with the large tubercles are indeed compound 

 after the diadematoid type). That the Pseudodiadematidce belong to the same group 

 I think nobody will doubt, even though the teeth are unknown. Further the Hemi- 



Fig. 4. Actinal side of a young 



Phormosoma placenta (3 mm. 



diameter) in the „I^rotecliinus"- 



stage. 



*) Wlien seelting information in „Rev. of Ecli." of the structure of tlie teetli in Cœiwpedina. I found 

 the following information: p. 292 „the teeth resemble those of Arbacia"; of the Arbaciidæ it is said 

 p. 2();i: „teeth like those of Diadcmatidæ and Cidaridæ" (unkeeled); p. 3!)i): „the teeth arc shaped as in 

 the Diadematidæ and Ecliinidie proper" (keeled and unkeeled); p. 688 the teeth arc said to be „keeled 

 in the middle". — It is ratlicr difficult to find; from these indications tlie real fact that in Arbaciida: 

 the teeth are keeled, in Cœnopedina they are unkeeled. 



