NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 55 



I. Division series composed of an irregular number of ossicles. 



i. The arms occupy only a portion of the distal border of the radials. 



i. All the arms are of equal length. 



i. All the arms terminate in a growing tip. 



1. All the arms are provided with ambulacral grooves. 



Pinnules 



i. Pinnules more or less sharply triangular in cross section. 



i. All of the pinnules similar. 



i. Pinnulation of the arm bases more or less deficient. 



i. Side- and covering-plates highly developed. 



i. All of the pinnules provided with ambulacral grooves. 



General 



2. Skeleton composed of less than a million ossicles. 



THE OCCURRENCE IN THE VARIOUS FAMILIES OF BOTH COM- 

 PONENTS OF CONTRASTING PAIRS 



Excepting for the Holopodidae, which is represented in the existing 

 seas by only a single species, all of the families of recent crinoids 

 agree in exhibiting, in closely related genera included in them, both 

 of the contrasted characters in a greater or lesser number of pairs. 



The number of entire pairs included in the various families is 

 apparently proportionate to the known recent representation of each 

 family. It is largest in the Pentacrinitidae, which includes by far 

 the greater part of all the existing types. 



In detail the contrasted pairs in each family are as follows : 



PENTACRINITIDAE 



I. Calyx 



The presence or absence of basals. 



The individual occurrence, or fusion, of the basals. 



The presence or absence of infrabasals. 



The occurrence of five or ten radials. 



The presence or absence of interradials. 



The presence or absence of anal x. 



The presence or absence of interbrachials. 



