NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 



43 



the arm bases became more or less crowded together, so that the 

 development of pinnules on the earlier brachials became impossible. 



With the further reduction of the calyx, which chiefly involved 

 the turning outward of the radials so that ultimately they attained 

 a position at right angles to instead of parallel with the dorsoventral 

 axis of the animal, the arms progressively became more and more 

 widely separated, and then, step by step, the proximal pinnules were 

 again able to develop. 



Although originally the crinoids possessed all of the proximal 

 pinnules, the primitive condition of the immediate ancestors of the 

 groups to which the recent types belong was a deficient pinnulation 

 of the arm bases. As the reappearance of the pinnules on the arm 

 bases was made possible by, and is therefore directly correlated with, 

 the more advanced stages in the reduction of the calyx, perfection 

 of the proximal pinnulation is in reality an evidence of specialization 

 through a reduction of the more fundamental elements of the 

 skeleton. 



Frequency at different depths Frequency at different temperatures 



< ^ f •* 



Degrees 



Fathoms 1 2 Fahrenheit 1 2 



O-IOO 2 2 8o-75 1 1 



I00-2OO 2 2 75-70 2 2 



200-300 3 i 70-65 2 I 



300-400 3 I 65-60 2 I 



4OO-5OO 3 I 60-55 2 I 



500-0O0 4 2 55-50 2 I 



600-700 4 2 50-45 2 I 



700-800 4 2 45-40 3 I 



80O-9OO 3 2 4O-35 4 2 



900-IOOO 3 2 35-30 2 I 



IOOO-I5OO 3 I 30-25 I I 



I500-2000 2 I 



2O0O-3000 2 I 



1 2 



Average depth 763 fathoms 722 fathoms 



Average temperature 50.7 ° Fahr. 52.9 Fahr. 



i. Side- and covering-plates highly developed. 



Bathymetric Thermal 



range range 



Pentacrinitidse (Calometridse, Tha- 



lassometridse, Charitometridae, 



and Pentacrinitida) 0-1600 34-2-75-7 



Apiocrinidse 565-940 36.7-38.1 



Bourgueticrinidae 62-2690 29.1-70.75 



Holopodidae 5-120 71.0 



Plicatocrinidae 266-2575 31. 1-43.9 



