536 



THE AMEBIC AN NATURALIST [Vol. XLIX 



and to the lower temperatures considerably greater than 

 it really is. 



In the subfamily Heliometrinse, the largest and most 

 widely distributed subfamily of the Antedonidse, which 

 itself is the dominant crinoid family of the present seas, 

 the range of temperature is very great; but as only one 

 out of the ten genera of the Heliometrinae is asymmetrical 

 it has seemed sufficient to consider and to tabulate the 

 temperature of this genus (Promachocrinus) alone. 



The frequency of the families including asymmetrical 

 crinoids at different temperatures is as follows : 



80-75 4 



60-55 : 



50-45 : 



40-35 I 



35-30 < 



30-25 : 



These frequencies fall into the following groups: 



30-45 4J 



45-65 3.5 



Considering the zone of optimum temperature (50°- 

 65°) in contrast to the temperatures above and below, and 

 omitting the exceptionally low temperatures below 30°, 

 we have: 



50-65 . 

 65-80 . 



