38 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



But in certain types the posterior arms (in the Palaeozoic usually 

 the anterior), particularly the left posterior, are more or less dwarfed 

 or atrophied, this resulting- in a more or less marked bilateral sym- 

 metry in which the anteroposterior axis may pass either through 

 the left posterior arm and the right anterior interambulacral area, 

 or through the anterior arm and the posterior interambulacral area. 



The dwarfing or atrophy of one or both of the posterior arms is 

 specialization through partial suppression of the normal arm devel- 

 opment. 



Frequency at different depths 



Frequency at different temperatures 



Fathoms 

 O-IOO 



100-200 



200-300 



300-400 



400-500 



500-600 



600-700 



700-800 



800-900 



900-1000 



1000-1500 



1500-2000 



2000-3000 



Average depth 



Average temperature 



Degrees 

 Fahrenheit 



80-75 



75-70 

 70-65 

 65-60 



60-55 

 55-So 

 5o-45 

 45-40 

 40-35 

 35-30 

 30-25 



822 fathoms 

 48.6 Fahr. 



359 fathoms 

 6 1. 4 Fahr. 



I. All the arms terminate in a growing tip. 



Bathymetric Thermal 



range range 

 Pentacrinitidse (except Comasteri- 



dse) 0-2900 28.7-80.0 



Apiocrinidae 565-940 36.7-38.1 



Phrynocrinidas 508-703 38.1-40.0 



Bourgueticrinidse 62-2690 29.1-70.75 



Holopodidaa 5-120 71.0 



Plicatocrinidse 266-2575 31. 1-43.9 



2. Some of the arms terminate in a pair of pinnules. 



Pentacrinitidse (Comasteridse) 



Bathymetric 

 range 



. . O-83O 



Thermal 

 range 



44.5-80.O 



Normally in the crinoids the arms grow continually throughout 

 life, and the arms therefore always terminate in a growing tip. 



