A. H. Clark — Groups of Recent Crinoids. 69 



The stalked groups, arranged according to their phylogenetic 

 position, have bathy metric ranges as follows: 



fathoms 



Pentacrinitidse 



2900 



Pentacrinitida 



1345 



Apiocrinidse 



375 



Phrynocrinidse 



44 4- 



Bourgueticrinidse 



2628 



Holopodidse 



115 



Plicatocrinidse 



2309 



It is evident that, while as a rule the more specialized fami- 

 lies possess small, and the less specialized large, bathymetric 

 ranges, there is no hard and fast line between the two groups ; 

 for we find the Jurassic Zygometridse with a range of only 153 

 fathoms, while the apparently very modern Bathy metrinse has 

 the greatest range of all, 2820 fathoms (fig. 1, lower border of 

 the black areas). 



So far as we know, pressure has no influence whatever upon 

 the distribution of the crinoids, which is determined primarily 

 by temperature ; thus the actual bathymetric range of any 

 crinoid species, genus or higher group means nothing unless 

 we know how large a temperature range it covers. 



For instance, the family Plicatocrinidse covers a range of 

 2309 fathoms, but the maximum variation in temperature 

 which it is known to withstand is 12*8° Fahrenheit, while the 

 subfamily Comasterinae has a bathymetric range of only 140 

 fathoms, but a thermal range of 27 , 7° Fahrenheit. 



The more specialized families possess small bathymetric 

 ranges solely for the reason that they occur in the littoral zone 

 where the isotherms are most numerous and most closely 

 crowded. 



For comparison with the table of contrasting groups given 

 above the following table, giving the temperature range of 

 each, is given : 



More specialized Less specialized 



Articulata 51-3° Fahr. Inadunata 12-8° Fahr. 



Pentacrinitidse 504 Bourgueticrinidae 41-6 



Comatulida 50-4 Pentacrinitida 35 -0 



Oligophreata 45-8 Macrophreata 50-4 



From this table it would appear that the thermal range (or 

 in other words the thermal adaptability) of the crinoid groups 

 increases with specialization. 



According to their average bathymetric ranges, that is, their 

 bathymetric ranges calculated as the average of the bathymetric 

 ranges of all the included genera, the crinoid families including 

 recent representatives arrange themselves as follows : 



