66 A. II. Clark — Study of Recent Crinoids. 



which have much greater total range, but lesser average range 

 and mean depth of habitat, and to those common to both 

 oceans, which have a much greater average range and mean 

 depth of habitat, as well as total range. 



In depth the maximum representation is between and 200 

 fathoms, especially between 50 and 150 fathoms. As, taking the 

 ocean as a whole, we find at a depth of 200 fathoms a temper- 

 ature of 50°'l,and at 100 fathoms 60 o, 7, it is evident that these 

 genera are most strongly represented within the optimum tem- 

 perature for crinoid life, which is between 50° and 65°. 



Temperature records are, unfortunately, few, and the table 

 showing the frequency of the genera at different temperatures 

 is, therefore, of uncertain value. But it is interesting to ob- 

 serve that, excepting Proisocrinus and Carpenterocrinus which 

 are only known from a single dredge haul each, between the 

 temperatures 36° and 40°, the only increase in the numbers 

 falls between 50° and 64°, that is, within the optimum temper- 

 ature for crinoids, and is particularly emphasized between 56° 

 and 64°, the emphasis within the optimum temperature range 

 being between 60° and 65°. 



