No. 595} 



A STUDY OF ASYMMETRY 



531 



Considering only the genera marked with an asterisk 

 (*) we find the representation at different depths given 

 in the tahle on page 530 and in Fig. 1. 



Considering the percentages only, these may be re- 

 grouped, as follows : 



The number of families of recent erinoids including 

 asymmetrical species, the number of families including 

 only symmetrical species, and the percentage of the num- 

 ber of families including only symmetrical species repre- 

 sented by the number of families including asymmetrical 

 species at different depths, are shown in the table on page 

 533 and in Fig. 2. 



The proportion of the genera including asymmetrical 

 species to those composed entirely of symmetrical spe- 

 cies, about one third between the shore line and 50 

 fathoms, decreases to a minimum of one tenth at from 300 

 to 350 fathoms, and then increases, with greater and 

 greater rapidity, to 1,600 fathoms and below. 



It is everywhere less than one quarter between 100 and 

 650 fathoms. Thus it is evident that the genera including 

 asymmetrical species are chiefly developed in shallow 

 water, and in deep water, and are least developed in water 

 of intermediate depth. 



Taking the ocean as a whole, the temperature at 100 

 fathoms is 60.7°, and at 650 fathoms 38.6°; the optimum 

 temperature for the recent erinoids appears to be between 

 50° and 65° ; when we remember that most of the asym- 

 metrical species, and all of the most asymmetrical ones, in 

 the genera which give us our numbers for 0-50 and for 

 50-100 fathoms, are confined to a littoral belt of scarcely 

 more than 50 fathoms, it becomes at once evident that 

 asymmetry among the erinoids is developed chiefly in 



