NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 21 



proximale. The so-called centrodorsal of the comatulids is such a 

 proximale below which the original column has been discarded. In 

 the pentacrinites the early growth is exactly as in the comatulids, 

 but the proximale never becomes attached to the calyx ; instead, new 

 columnals are formed between it and the crown and a new stem is 

 formed for which the original proximale serves as a terminal stem 

 plate, and a second proximale appears beneath the calyx ; this process 

 continuing, a series of so-called nodals is formed, each of which 

 represents, so to speak, an attempt of the column to limit itself to a 

 definite length and to cease all further growth. In the Bourgueti- 

 crinidse, and in most of the Apiocrinidse, the original proximale is 

 reduplicated, so that just beneath the calyx there is found a more or 

 less conical structure composed of a series of proximales which 

 increase in perfection to that just beneath the calyx. 



The abrupt limitation in the growth of the column, and the forma- 

 tion of a proximale which becomes rigidly attached to the calyx, pre- 

 venting the formation of additional columnals between it and the 

 calyx, in contrast to the primitive method of continuous stem growth 

 during life, is specialization through inhibition and definite limita- 

 tion of the skeleton forming power of the column. 



Frequency at different depths Frequency at different temperatures 



r~ * ■» t A -< 



Degrees 

 Fathoms 1 2 Fahrenheit 1 • " 2 



O-IOO O 2 80-75 O I 



I00-2OO 2 75-70 2 



2OO-3OO I 2 ' 70-65 2 



3OO-4OO I 2 65-60 2 



400-500 I 2 60-55 2 



5OO-60O ' 2 3 55-50 2 



600-700 i 3 50-45 2 



700-800 1 3 45-40 1 2 



800-900 1 2 40-35 2 4 



900-1000 1 3 35-30 1 2 



IO0O-I500 I 2 30-25 O 2 



I50O-2OOO I 2 



2O00-300O I 2 



1 2 



Average depth 004 fathoms 797 fathoms 



Average temperature 37-5° Fahr. 50. 1° Fahr. 



I. Column terminating in an expanded terminal stem plate. 



Bathymetric Thermal 



range range 



(Pentacrinitidae) 0-2900 28.7-80.0 



Apiocrinidae 565-940 36.7-38.1 



Phrynocrinidse 508-703 38.1-40.0 



Holopodidae ■ 5-120 71.0 



Plicatocrinidae 266-2575 31. 1-43.9 



