NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 



41 



slender ; and hence we may look upon this change as correlated with 

 an increasing suppression of the skeleton forming power. 



Frequency at different depths 



Fathoms 



O-IOO 



100-200 



200-300 



3OO-4OO 



4O0-5OO 



500-600 



60O-70O 



700-800 



80O-9OO 



9OO-IOOO 



IOOO-I5OO 



I5OO-20O0 



2OOO-3OOO 



Frequency at different temperatures 



r * 



Degrees 

 Fahrenheit 



8o-75 

 75-70 

 70-65 

 65-60 

 60-55 

 55-50 

 50-45 

 45-40 

 40-35 

 35-30 

 30-25 



Average depth 754 fathoms 



Average temperature 50.4° Fahr. 



808 fathoms 

 52.5 ° Fahr. 



I. All of the pinnules similar. 



Bathymetric 

 range 



Thermal 

 range 



Pentacrinitidse (Ptilometrinje, Ate- 



lecrinidas, Pentacrinitida) 0-1350 



Apiocrinidse 565-940 



Phrynocrinidae 508-703 



Bourgueticrinidse 62-2690 



Holopodidse 5-120 



Plicatocrinidas 266-2575 



2. The proximal pinnules modified. 



Bathymetric 

 range 



Pentacrinitida^ (Comatulida, except 



Ptilometrinse and Atelecrinidae) . 0-2900 



36.0-80.0 



36.7-38.1 



38.1-40.0 



29.1-70.75 



71.0 



3i 1-43-9 



Thermal 

 range 



28.7-80.O 



So far as we know the earlier crinoids, like the young comatulids 

 before the appearance of P x , had all the pinnules similar, except in 

 the cases in which the proximal segments of the lower pinnules were 

 embedded in the calyx wall, when these were enlarged and broadened. 



But in the dominant types to-day the proximal pinnules are almost 

 always modified, having lost their original significance and adopted 

 instead the function of slender tactile or stout protective organs. 



The modification of the proximal pinnules is always associated 

 with the loss not only of the ambulacral grooves and the associated 

 structures, but also of the genital organs ; and it therefore is possible 



