NO. 7 SEA-LILIES AND FEATHER-STARS — CLARK " 21 



arms are never more than lo in number except in the forms with lo 

 radials, which may have lo or 20. 



A careful study of these types, together with a detailed comparison 

 between their characters and the characters shown by other species 

 found in water abnormally cold for their immediate phylogenetic 

 stock shows that the apparently primitive characters are without 

 doubt the result of a repression or an inhibition of the normal 

 phylogenetic development ; furthermore, in combination with these 

 characters we always find other characters which indicate a condition 

 of very marked specialization, as for example a large number of 

 proximal pinnules which are provided at their tips with more or less 

 developed terminal combs, both of which characters are otherwise 

 only found in the comatulids of very warm water at the opposite end 

 of the temperature scale, and a great specialization of the centro- 

 dorsal. 



The species of very cold vv^ater thus resemble the species of very 

 warm water in the possession of a fundamentally aberrant structure, 

 for they preserve and exaggerate certain very primitive characters 

 while at the same time they show a high degree of specialization along 

 other lines. 



However extraordinary it may appear, in their unbalanced type of 

 specialization the comatulids of the coldest water agree more nearly 

 with the species inhabiting very warm water — nearly all of which 

 belong to the other suborder — than with any of the species of the 

 intermediate waters, and the largest species, the smallest species, and 

 the species with the greatest number of segments in the arms, pinnules 

 and cirri are found equally at both extremes of temperature. 



The Oligophreata and the Macrophreata are both represented by 

 six families between the temperatures of 50° and 55° F., but the 

 Oligophreata predominate at all temperatures above this, while the 

 Macrophreata predominate at all temperatures below. 



In the Oligophreata the greatest number of families is found be- 

 tween 60° and 65°, and in the Macrophreata between 50° and 66°. 

 For all the comatulids the greatest representation is between 55° and 

 65°, with the emphasis on 60° -65°. 



It would therefore appear that the temperature range included 

 between 55° and 65° represents the temperature phylogenetically 

 most suitable for the recent crinoids. It is a very curious fact that 

 the comatulids found between 55° and 65° are all of medium size, 

 none very large and none very small, and that they all show well- 

 balanced and conservative characters. 



