THE INORGANIC CONSTITUENTS OF THE SKELETONS 



OF ZYGOMETRA MICRODISCUS AND OF CHONDROMETRA RUGOSA. 



At my request Professor Frank W. Clarke of the United States Zoological Survey 

 made detailed analyses of fragments of the arms of a specimen of Zygometra microdiscus 

 (Bell) from the Aru Islands in 13 metres of water (Stat. 273), and also of a specimen of 

 Chondr ometra rugosa (A. H. Clark) from the southern coast of Timor in 520 metres (Stat. 297). 



It is a very curious fact, first discovered by Professor Clarke in making a series of 

 analyses for me, that in the crinoid skeleton the proportion of magnesium carbonate (MgCO g ), 

 which is always found in conjunction with the calcium carbonate (CaCA 8 ), varies according to 

 the temperature of the habitat, quite regardless of the systematic position of the animals. Thus 

 in the skeletons of the crinoids living in water having a temperature of from — i.8° to 

 + i-5° C. the percentage of magnesium carbonate is from 7.28 to 8.23, while in the littoral 

 crinoids of the Philippine Islands the percentage is from 12.20 to 12.69. In water of inter- 

 mecliate temperatures intermediate percentages are found. 



This holds good not only for the crinoids, but for all other echinoderms as well, and 

 indeed apparently for all other marine animals which have a strictly internal calcareous skeleton - 

 alcyonarians, most foraminifera, etc. 



The actual analyses of the skeletons of Zygometra microdiscus and of Chondrometra 

 rugosa, which were made by Mr. W . C. Wheeler under the direction of Professor Clarke, 

 gave the follpwing results : 



Zygometra microdiscus 



Chondrometra rugosa 



SiO., 



R 2 3 ') 



MgO 



CaO 



PA 



Loss on ignition 

 CO., needed . . 



0.04 

 0.48 

 4.92 



37-19 

 0.17 



55-OS 

 3447 



0.05 

 0.23 



3-99 

 42.72 



Tracé 



51.69 



37-95 



1) The symbol "R.2O3" represents the sum of ferric oxide and alumina, and "Loss on ignition" covevs carbon dioxide, water 

 and organic matter. 



The C0. 2 calculated to satisfy the bases is given. 



