GEOCHEMISTRY: CLARKE AND KAMM 
403 
In such a small specimen the unavoidable errors of manipulation be- 
come serious, for the reason that they are magnified in the calculations. 
In this instance even so small an error in weighing as 0.1 mgm. would 
amount to 0.3 of 1% in the final statement. The material for analysis 
6 was also hardly adequate. These two analyses, however, are not 
worthless, for they show the high percentage of magnesium carbonate 
which characterizes the organisms from tropical waters; and so too do 
the others. 
The two analyses of sea urchins show less magnesium carbonate than 
was found in the other echinoderms, and so help to confirm our suspicion 
that as a group these animals are poorer in magnesia than either the 
crinoids, the starfishes, or the brittle stars. In the previously published 
analyses of crinoid skeletons, the highest proportion of magnesium car- 
bonate found was 13.37%, in a specimen from the Aru Islands. The 
one from Tobago is notably richer. As for the starfishes and brittle 
stars the maximum figure previously found for magnesium carbonate 
was 14.08% in a specimen from Culebra; a quantity which is exceeded 
by three analyses in our new series. 
The relation between magnesium carbonate and temperature is strik- 
ingly illustrated by four new analyses of starfishes, which we append 
here for comparison with the Tobago series. They are as follows : 
1. Pontaster tenuispinus, Verrill. Albatross Station No. 2095, between 
Cape Hatteras and Nantucket. Latitude, 39°.29'.00" N. Longitude 
70°.58'.40" W. Depth of water, 2456 meters. At that depth the tem- 
perature must have been low; not far from 0°C. 
2. Plutonaster agassizii, Verrill. Collected off Marthas Vineyard, 
Massachusetts. Depth of water, 584 meters. Bottom temperature 
6.6°C. 
3. Odontaster hispidus Verrill. Collected off Marthas Vineyard. 
Depth, 245 meters. Bottom temperature 11.1°C. 
4. Astropecten articulatus Say. West Coast of Florida. 
1 
2 
3 
4 
SiOz 
0.31 
0.35 
0.62 
0.24 
(Al,Fe)203 
0.45 
0.18 
0.12 
0.23 
MgCOs 
8.86 
9.09 
10.58 
13.02 
CaCOs 
89.34 
89.18 
87.16 
85.08 
CaS04 
0.89 
1.20 
1.44 
1.43 
CasPzOa 
0.15 
0.08 
trace 
100.00 
100.00 
100.00 
100.00 
