NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 33 



resultant loss of half of the number of arms, is an illustration of 

 specialization through suppression. 



Frequency at different depths Frequency at different temperatures 



Degrees 

 Fathoms 1 2 Fahrenheit 1 2 



O-IOO 2 2 80-75 I ° 



I00-2O0 2 2 75-70 2 I 



20O-3OO I 2 70-65 I I 



3OO-4OO I 2 65-60 I I 



400-500 I 2 60-55 I I 



500-6OO 2 2 55-50 I I 



60O-70O 2 2 50-45 I 2 



700-800 3 I 45-40 I 2 



800-900 • 3.1 40-35 3 2 



QOO-IOOO 3 I - 35-30 -2 I 



1 000-I500 2 I 30-25 I 



I500-200O 2 



2O00-300O 2 O 



1 2 



Average depth 865 fathoms 483 fathoms 



Average temperature 50.5° Fahr. 50.0 Fahr. 



i. The first bifurcation is at a more or less indefinite distance 

 beyond the second post-radial ossicle. 



Bathymetric Thermal 



range range 



Pentacrinitidse (Metacrinus) ...... 30-630 39-5~7i-0 



Phrynocrinidse 508-703 38.1-40.0 



Plicatocrinidse 266-2575 31.1-43.9 



2. The first bifurcation is on the second post-radial ossicle. 



Bathymetric Thermal 



range . range 



Pentacrinitidse (except Metacrinus) 0-2900 28.7-80.0 



Apiocrinidae 565-940 36.7-38.1 



Bourgueticrinidae 62-2690 29.1-70.75 



Holopodidse 5-120 71.0 



In the earlier crinoid types, especially before the formation of 

 definite IBr series, the bifurcation of the arms commenced at a more 

 or less indefinite distance from the radials. 



Later the number of plates intervening- between the radials and 

 the first axillary became reduced to one only. 



The reduction of the number of plates between the radials and the 

 first axillary from several to one only indicates specialization through 

 the suppression of useless skeletal structures. 



