300 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Professor Clarke says that " in order to make these analyses more instructive 

 it is necessary to recalculate them into such form as to show the composition of 

 the true crinoid skeleton that is, to eliminate the highly variable organic matter 

 of the original specimens. On doing this, and recalculating to 100 per cent, we 

 find that they assume the following form " : 



Table of revised analyses. 



1 Recalculated from the figures given above. 



Professor Clarke notes that from the figures it is perfectly clear that the pro- 

 portion of magnesium carbonate in the crinoid skeleton is in some way dependent 

 on the temperature of the habitat, being least in the crinoids occurring in very 

 cold water and, increasing with extraordinary regularity, greatest in those found 

 in the tropical littoral. 



At first Professor Clarke supposed that the peculiar relation between the 

 temperature of the habitat and the composition of the skeleton might possibly be 

 due to a difference in the form of the most abundant carbonate the less stable 

 aragonite in the warm- water forms, and calcite in the crinoids from colder regions. 

 But tests by Meigen's reaction proved that the organisms were all calcitic, and so 

 this supposition had to be abandoned. 



In considering the composition of the crinoid skeleton it should be borne in 

 mind that the structure is always entirely internal, protected from the surrounding 

 water by living tissue, so that whatever alteration it may undergo after its original 

 deposition can not be influenced except very indirectly by the water in which the 

 animal lives. 



