NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 15 



1. Anal x, bearing a process, present. 



Bathymetric Thermal 



range range 



Promachocrinus 10-222 28.7 



Thaumatocrinus 361-1800 37.4-42.7 



2. Anal x absent. D .. -, , 



Bathymetric Thermal 



range range 



Pentacrinitidse (except Promacho- 

 crinus and Thaumatocrinus) .... 0-2900 28.7-80.0 



Apiocrinidse 565-940 36.7-38.1 



Phrynocrinidae 508-703 38.1-40.0 



Bourgueticrinidas 62-2690 29.1-70.75 



Holopodidse 5-120 71.0 



Plicatocrinidse 266-2575 31. 1-43.9 



The problem of the so-called interradials in Promachocrinus and 

 Thaumatocrinus is a very complicated one. 



These five interradial radials arise as five simple interradials, cor- 

 responding- exactly to the interradials of many fossil forms, and that 

 in the posterior interradius gives rise to a process, being the homo- 

 logue of the anal x of fossil types. 



These interradial radials being primarily interradials, and the one 

 in the posterior interradius being the representative of anal x, it 

 naturally follows that the forms in which they occur present a more 

 primitive type of structure, more nearly similar to the ancient struc- 

 tural types, than those from which they are absent as a result of the 

 progressive simplification of the skeleton by the gradual suppression 

 and elimination of superfluous calcareous elements. 



But on the other hand these interradial radials do not retain the 

 status of simple interradials. They grow to an equal size with the 

 true radials, and each gives rise to a post-radial process which, start- 

 ing as a simple linear series of ossicles, eventually comes to be exactly 

 like that arising from the true radials. 



This type of structure is quite unique, and may therefore be con- 

 sidered as an evidence of specialization. 



Hence the five interradial radials of Promachocrinus and Thauma- 

 tocrinus must be considered, if viewed in the light of their origin, as 

 indicating a low degree of specialization marked by the retention of 

 the primitive interradials, and of anal x; but if viewed in the light of 

 their ultimate condition, as indicating a high degree of specialization. 



