Studies on tlie Ecliinoidea Holectypoida. 



221 



■ofPshoot from the Orthopsid stock, bridging the gap between 

 Endocyclica and Exocyclica, but leading nowhere. But in view of 

 the evidence at present available, the possibility of a direct sequence 

 to Galernpi/gus seems equally likely. If further material, more 

 satisfactorily preserved, should be forthcoming, the latter alternative 

 may well appear jDreferable. And, in such an event, a radical 

 change in classification of Irregular Echinoids would result. The 

 whole " Exocyclic " series would prove diphyletic, the Holectypoida 

 and Nucleolitoida being divergent stocks, practically synchronous 

 in appearance, but quite distinct in origin. Such is the interpreta- 

 tion that I incline to give of the new type here described ; but 

 without better material the following scheme must be regarded as 

 purely tentative and in some measure hypothetical. 



Diagram op Probable Lines of Descent among Irregul.4r 



Echinoids. 



Holectypoida 



Nucleolitoida 



CLyPEASTROIDA 



Recent and 



Tertiary. Cassiduloida'* 



Centrechinoida 



Spatangoida 



Cretaceous. 



ClYPEIDjE 



Oolitic. 



Liassic. 



Ple^echinus 



^COLLYRITIDiE 



Galekopygus 



DiADEMOPSIS PSEUDOPYGASTER 



Triassic. 



Mesodiadema 

 EXPLANATION OF PLATE IX. 



FIG. 

 1.- 



2.- 

 3.- 



4.- 

 5.- 

 6.- 



7.— 



-Pseztdopygaster eos (ITolotype), adapical view. 



■P. eos, adoral view. 



-P. eos, posterior view. 



-P. eos, two interradial plates below the ambitus. 



-P. eos, part of Amb. I almost on ambitus. 



-P. eos, an interradial plate above the ambitus. 



-Mesodiadema marconissae (after de Loriol). 

 8. — M. latum (after Bather). 



9. — Galeropygus agariciformis. Adoral surface. (The " knobs " on the inter- 

 radial carinae are slightly over-emphasized.) 



Postscript. 



With this note the series of papers under the title " Morphological 

 Studies on the Echinoidea Holectypoida and their Allies " is brought 

 to conclusion. This does not imply that I have written all I hope 



