NO. 10 PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 39 



But in case arm growth is arrested it frequently happens that a 

 definite perfected arm type is acquired which terminates in a pair of 

 pinnules and is capable of no further development. 



The presence of (posterior) arms terminating in a pair of pinnules 

 indicates specialization through more or less extensive suppression of 

 the normal arm growth. 



Frequency at different depths Frequency at different temperatures 



( ' > I * N 



Degrees 

 Fathoms 1 2 Fahrenheit 1 2 



0-100 3 i 80-75 r J 



100-200 3 I 75-70 3 I 



200-300 2 I 70-65 2 I 



300-400 3 I 65-60 2 I 



400-500 3 1 60-55 2 I 



500-600 5 1 55-50 2 i 



600-700 5 1 50-45 2 I 



700-800 5 1 45-40 3 i 



800-900 3 1 40-35 5 o 



900-1000 3 o 35-30 2 o 



1 000-1500 2 o 30-25 2 



1500-2000 2 o 



2000-3000 2 O 



1 2 



Average depth 723 fathoms 450 fathoms 



Average temperature 42.9 Fahr. 60.0 Fahr. 



i. All the arms are provided with ambulacral grooves. 



Bathymetric Thermal 



range range 



Pentacrinitidse (except Comasteri- 



dse) 0-2900 28.7-80.0 



Apiocrinidce 565-940 36.7-38.1 



Phrynocrinidse 508-703 38.1-40.0 



Bourgueticrinidse 62-2690 29.1-70.75 



Holopodidae 5-120 71.0 



Plicatocrinidse 266-2575 31. 1-43.9 



2. The posterior arms are without ambulacral grooves. 



Bathymetric Thermal 



range range 



Comasteridce 0-830 44.5-80.0 



In the crinoids all of the arms are normally provided ventrally 

 with ambulacral grooves. 



From the posterior arms of certain types, which always are correla- 

 tively more or less dwarfed, these ambulacral grooves may be absent. 



The absence of ambulacral grooves on the posterior arms (which 

 may involve as many as three of the five radii) indicates specializa- 



