NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 



23 



Combined with a broad spreading- base composed of a mass of 

 swollen, distorted and overgrown columnals, the early crinoids com- 

 monly possessed stout and massive radicular cirri, which were very 

 irregular in position, and equally irregular in structure. In the 

 Articulata this type of stem base occurs only in fossil species belong- 

 ing to the family Apiocrinidse, though a suggestion of it is found in 

 the young of certain macrophreate comatulids, particularly those 

 belonging to the genus Hathrometra; elsewhere one or other of the 

 root systems has been suppressed. 



The presence of radicular cirri appears to be of the same funda- 

 mental significance as the presence of a terminal stem plate, and 

 therefore any type without it must be considered as possessing a 

 highly specialized type of column. 



The absence of radicular cirri, just as the absence of a terminal 

 stem plate, indicates specialization through suppression of a funda- 

 mentally important skeletal structure. 



The recent crinoids possess either radicular cirri or a terminal 

 stem plate, but never both combined as do many of the earlier types ; 

 one or the other is always suppressed. As the suppression of either 

 is equally an evidence of specialization, it naturally follows that we 

 have here, in the presence or absence of the radicular cirri and the 

 correlated absence or presence of the terminal stem plate, two cate- 

 gories each of which is the complement of the other, while both repre- 

 sent an equivalent stage in phylogenetic advancement. 



Frequency at different depths 



Frequency at different temperatures 



Fathoms 



O-IOO 



100-200 



2OO-3OO 



3OO-4OO 



4OO-5OO 



50O-OOO 



60O-/OO 



70O-8OO 



80O-9OO 



900-1000 



IOOO-I50O 



I5OO-20OO 



2OO0-3O0O 



Average depth 



Average temperature 



