MONOGRAPH OF THE EXISTING CRINOIDS. 301 



It is interesting to note that the compactness of the crinoid skeleton increases 

 in direct proportion to the percentage of magnesia in its composition. 



Professor Clarke remarks that the proportion of magnesia given for Ilathro- 

 metra tenella is probably too low, for the specimen as analyzed contained over 6 

 per cent of silica and sesquioxides — evident impurities due to adherent mud from 

 which the delicate structure could not be wholly freed. If these are rejected, the 

 magnesium carbonate (MgCOj) is raised from 9.36 to 10 per cent, which gives the 

 crinoid a better and more probable rating. 



This line of investigation was subsequently extended by Professor Clarke to in- 

 clude all the echinoderm groups, and of the echinoderms as a whole he says : 



From the evidence now avallnble It seems almost certain that the Inorganic constituents of 

 any echinoderm will have the composition of a moderately magnesian limestone. There may be 

 exceptions, but none hits as yet been found. The three tables, for crlnoids, sea urchins, and 

 starfishes [including ophlurans], all tell the same story, and with remarkable unanimity. 

 Furthermore, the proportion of magnesium carbonate appears to be a function of temperature, the 

 organisms from warm regions being richer In It than the cold-water forms. The exceptions 

 to this rule are apparent rather than real ; for cold or warm currents and varying depths of 

 water account for all seeming irregularities. The sea urchins seem to be a little poorer In 

 magnesia than either of the other groups, but the analyses are fewer and therefore less con- 

 clusive. Silica and sesquioxides are probably altogether extraneous, although it Is possible 

 that small quantities of them may really l)elong to the organisms. In phosphate of lime the 

 starfishes are richest, and all the specimens analyzed contain It In small amounts. Whether 

 it is an essential constituent or not is uncertain. As shown by Melgen's reaction, all the 

 echinoderms studied are ealcitic, and no evidence of aragonite io them was found. 



The temperature regularity shown by the analyses offers an interesting biological problem 

 with which we can not undertake to cope. It is not due to differences of composition In the 

 solid matter of sea water, for that is practically uniform all the world over. In all the great 

 oceans, and even in minor bodies of water like the Mediterranean, the Baltic, and the Black 

 Sea, the proportion of magnesia to lime is very nearly if not actually constant In gaseous con- 

 tents, and especially in carbon dioxide, the waters vary, the gases being more soluble in cold 

 than In warm water. Whether this fact has any relation to the phenomenon under discussion 

 we can not attempt to say. We can onl.v report the facta and leave their biological discussion 

 to others. 



On the geological bearing of the evidence now before us it is easy to speculate; but here 

 great caution is needed. It would be unwise to assume that magnesian sediments are more 

 abundantly deposited in warm than in cold climates, and so to develop a system of what might 

 be called palseoclimatology. Against such an attempt there are two obvious reasons. First, 

 the sediments are only in small part derived from echinoderm remains. Other agencies are 

 more important in the formation of marine limestones. Secondly, a dense population, so to 

 speak, of cold-water organisms may deposit much more magnesia than a sparse population of 

 warm-water forms. The data now In hand, with all their suggestiveness, are too few to warrant 

 any far-reaching generalizations. It is our intention to carry the Investigation still further 

 studying other marine invertebrates by the same methods as those which we have followed here. 



More recently Professor Clarke, in connection with Dr. E. M. Kamm, has pub- 

 lished an analysis of a specimen of Tropiometra picta collected at Pigeon Point, 

 Tobago, in shallow water, of which the temperature was about 28° C The analysis 

 follows : 



SIC (Al,Fe).Oi MgCO, CaCO. CaJ>,0. CaSO. 

 0.54 0.51 1.3.74 83.13 0.64 1.44=100.00 



