CONCERNING THE STRUCTURE OF AGELACRINITES 



AND STREPTASTER, EDRIOASTEROIDEA OF THE 



RICHMOND AND MAYSVILLE DIVISIONS OF 



THE ORDOVICIAN. 



Stephen R. Williams. 



CONTENTS. 



Introduction and Sources of Material 59 



Historical Review, arranged chronologically 62 



Structure: 



Peripheral ring 65 



Cover plates from oral side 67 



Peristomial plates from oral side 69 



Peristomial plates and substomial chamber from aboral side 70 



Flooring plates 72 



Aboral ends of cover plates 73 



Rate of growth 74 



Possibility of locomotion 76 



Respiration, discussion of anal pyramid 76 



Water system 77 



Stages in development 78 



Evidence of alimentary canal •' 79 



Summary and conclusions 80 



Bibliography 81 



Descriptions of plates 83 



Plates, 1-9 87-95 



Introduction and Sources of Material. 



The study of living animals gives many clues to the life 

 history of closely related forms found fossil. There are, how- 

 ever, groups of fossil animals which have no present-day 

 representatives and these groups are far less easy to understand. 

 . The Cystids are a division of the Echinoderms which dis- 

 appeared completely with the end of the Palaeozoic. The study 

 of any cystid can, therefore, be aided only by distant analogies 

 from the groups most closely related which are still in existence. 

 Of the four divisions of the Echinoderms now extant the 

 Crinoids are the most primitive and therefore should give most 

 light on extinct forms. 



I wish to attempt a study of one of the longest lived of the 

 Cystids, Agelacrinites, Vanuxem. This genus is found from 

 the Ordovician to the Carboniferous inclusive, according to 

 Zittel, and the time of its greatest abundance is the Ordovician. 

 The forms found in the Richmond and Maysville divisions of 

 the Ordovician will be the only ones considered in this study. 



59 



