Tin^: ixo]<(;axic constituents of tup: skitthons 
OF ZYGUMI'lTRA MICRODISCUS AND OF CHOXl )R0.MI':TRA RUGOSA. 
At my request Professor Frank W. Clarkk of the United States Zoological Survey 
made detailed analyses of fragments of the arms of a specimen of Zygometra inicrodisciis 
(Bell) from the Aru Islands in 13 metres of water (Stat. 273), and also of a specimen of 
CJtondrouictra riigosa (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 (MgCOj), 
which is always found in conjunction with the calcium carbonate (CaCA.;), varies according to 
the temperature of the habitat, (juite regardless of the systematic position of the animals. Thus 
in the skeletons of the crinoids living in water having a temperature of from — 1.8° to 
+ 1.5° C. the percentage of magnesium carbonate is from 7.28 to iS.23, while in the littoral 
crinoids of the Philippine Islands the percentage is from 12.20 to 12.69. J" water of inter- 
mediate 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 inicrodisciis and of Choiidrotnctra 
rugosa, which were made by Mr. W. C. Wheeler under the direction of Professor Clarke, 
gave the following results : 
Zygometra microdisciis 
Choiidrotnctra rus,osa 
SiO, 
R,0, ') 
MgO 
CaO 
p,o, 
Los.s on ignition 
CO., needed . . 
0.04 
0.48 
4.92 
37-I9 
0.17 
55-05 
34-47 
0.05 
0.23 
3-99 
42.72 
Trace 
51.69 
37-95 
l) The symbol "RjOj" represents the sum of ferric oxide .ind alumina, and "Loss on ignition" covers carbon dioxide, «Mter 
and organic matter. 
The COj calculated to satisfy the bases is given. 
