No. 5S5J A STUDY OF ASYMMETRY 



535 



have an increase between 350 and 500 fathoms — that is, 

 more or less coinciding with this minimum. 



In the frequency of families at different depths ex- 

 pressed as percentages of the total number we notice a 

 minimum between 350 and 500 fathoms which readies a 

 point not again touched until 750-800 fathoms and beyond. 



This indicates the occurrence here of a proportion- 

 ately large number of families including asymmetrical 

 species, but at the same time a proportionately small 

 number of genera including asymmetrical species within 

 those families. 



Thekmal Distbibution of the Asymmetbical Cbixoids 

 In examining the thermal distribution of asymmetry 

 among the recent crinoids we find it advisable to employ 

 family instead of generic units, for the reason that our 

 records are insufficient to furnish us with even approxi- 

 mate thermal ranges for many of the individual genera, 

 though in most cases these may be estimated with rea- 

 sonable exactness. The records for the crinoids of the 

 deeper water are far more satisfactory than the records 

 for the crinoids of the littoral, and this is very fortunate, 

 for it justifies us in assigning a temperature of 65° and 

 over to a number of species and genera which are of 

 great importance in the present study. 



In considering asymmetry in relation to temperature 

 by family units it must constantly be borne in mind that, 

 whereas certain families (Capillasterinae, Comactiniinse 

 and Comasterime) extend from the warm littoral into 



is strongly marked only in llry shallow water of high tem- 

 perature, and is only slightly marked— indeed not infre- 

 quently entirely absent, as in Comatilia — in genera and 

 species inhabiting deep and cold water. 



Thus through a study of family units the amount of 

 asymmetry shown at intermediate temperatures is really 

 exaggerated, and appears in its relation to the higher 



