402 
GEOCHEMISTRY: CLARKE AND KAMM 
poorer. In specimens from high latitudes or from very deep water the 
proportion of magnesium carbonate ranged from 7 to 10%, while those 
from tropical waters contained from 11 to over 14%. Intermediate 
conditions of environment gave intermediate values. 
This strange relation between composition and temperature naturally 
suggested to us the desirabiUty of comparing a series of echinoderms 
from one definite locality; and an opportunity to do so fortunately pre- 
sented itself. An expedition sent by the Carnegie Institution of Wash- 
ington, to the island of Tobago in the British West Indies, made a large 
collection of just such material as we needed; and Dr. Hubert Lyman 
Clark, who had charge of the echinoderms, kindly furnished us with 
specimens. All of them were collected at Pigeon Point, in shallow water 
along shore, and at a temperature of about 28°C. That is, all the speci- 
mens came from exactly the same environment, and were, therefore, 
for our purpose, strictly comparable. The Kst of them is as follows: 
1. Mellita sexiesperforatus, Leske. A sea urchin. Adult. 
2. Echinometra lucunter, Linne. A sea urchin. Half grown. 
3. Tropiometra carinata, Lamarck. A crinoid. Adult. 
4. Asterina minuta, Gray. A starfish. Large adult. Weight 0.1009 
gramme. 
5. Linckia guildingii, Gray. A starfish. Young. 
6. Ophiocoma pumila, Lutken. A brittle star. Adult. 
7. Ophiomyxa fiaccida, Say. A brittle star. Half grown. 
8. Ophiomyxa flaccida. Adult. 
The analyses, reduced to uniformity by rejection of organic matter 
and water, and recalculation to 100%, are as follows and represent inor- 
ganic skeletal matter only. 
1 
2 
3 
4 
5 
6 
7 
8 
Si02 
0.15 
0.13 
0.54 
1 0.70 
0.24 
1 0.47 
0.66 
0.16 
(Al,Fe)203.... 
0.39 
0.37 
0.51 
0.26 
0.85 
0.70 
MgCOa 
11.91 
11.56 
13.74 
12.53 
14.31 
12.97 
14.95 
14.56 
CaCOs 
85.02 
83.87 
83.13 
86.77 
83.42 
84.44 
79.37 
81.02 
Ca3p208 
trace 
1.85 
0.64 
trace 
trace 
0.14 
trace 
trace 
CaS04 
2.53 
2.22 
1.44 
? 
1.77 
1.98 
4.17 
3.56 
100.00 
100.00 
100.00 
100.00 
100.00 
100.00 
100.00 
100.00 
In these analyses the silica and the sesquioxides are doubtless impuri- 
ties, due to adherent sand or mud from which the specimens could not 
be wholly freed. Analysis -4, of Asterina, is incomplete; for the amount 
of material, only 0.1009 gram, was insufficient for thorough work. 
