NO. 10 PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 



13 



Primarily the infrabasals occur as five small plates within (or below) 

 the basal ring, and alternating in position with the basals. 



Although potentially present in all the recent families of the Articu- 

 lata, at best the infrabasals are represented by insignificant plates, 

 invisible exteriorly, in the adults, while usually they are represented 

 only in the very young, or are absent altogether. 



The progressive suppression and final elimination of the infra- 

 basals, plates which, so far as we know, are of fundamental impor- 

 tance in the Articulata, is directly correlated with the specialization 

 of the respective types. The Plicatocrinidse, which belong not to the 

 Articulata but to the Inadunata, represents probably a primarily 

 monocyclic type. 



Frequency at differen 



depths 



Frequency at different temperatures 



