i65 



NEW YORK STATE MUSEUM 



Invagination of body wall f between regular 



thecal plates 

 (Endospires) (Simple hydrospires) 



at corners Goniospires 



at plate margins 



Craspedospires 

 Subtypes variously 

 protected. 



between thecal plates f flowing orad Anaspires 

 associated with food I 

 ^ grooves 



(Flowing hydrospires) 



flowing aborad Cataspires 



Evagination of body wall { Papulae [ forking and 



(Exospires) 



Podia 



I epithecal " Pectinirhombs " or epi- 



I spires. 



[ simple as in Cliocrinus. 



None of the more complex types of this chart exist today, while 

 simple exospires in the form of papulae are abundant. The forms 

 with endospires developed in regions of remarkably clear waters, 

 that is, waters free from lutaceous material derived from land sur- 

 faces or from volcanic ash. The simple hydrospires no doubt pos- 

 sessed ciliated fold-surfaces, but while simple reef muds (Hydro- 

 calcilutites) could be easily removed from their folds by solution, 

 any marked amounts of silicious sediments in the water would be 

 likely to obstruct such folds and kill the organisms possessing them. 

 Paths of melting icebergs and areas off river mouths would be likely 

 to interfere with distribution. Changing conditions, such as the 

 presence of a glacial period, would then lead to extinction of species 

 over large areas. Among endospires, the flowing type was the 

 most successful and anaspires persisted until Permian times. Forms 

 retaining the simple or unspecialized types of exospires best sur- 

 vived the trials besetting them in past ages and gradually became 

 the dominant types of today. 



