'HE INORGANIC CONSTITUENTS OF .MARINE INVERTEBRATES. 



By Frank Wioglesworth Ci,arke and Walter Calhoun Wheeler. 



INTRODUCTION. 



That many rocks were once marine sediments and that animals and plants shared in tlieir 

 formation is one of the commonplaces of geology. Coralline limestones, shell limestones, and 

 crinoidal limestones are among the most familiar illustrations of this statement. That corals 

 and shells furnish calcium carbonate to the sediments, that radiolarians and diatoms are siliceous, 

 and that vertebrate animals, some crustaceans, and a few brachiopods are more or less phos- 

 phatic are also well-known facts, which, however, have been determined in a more or less desul- 

 tory way. No systematic investigation to ascertain just what substances each class of organisms 

 contributes to the sediments seems to have been made, and the present research is an attempt 

 to cover the ground a little more thoroughly than it has been covei'ed heretofore. Complete- 

 ness is, of course, impossible, but something of a foundation for future work we have tried to lay. 



Although many analyses of corals and molluscan shells are on record, some important 

 groups of organisms have received only slight attention, and little has been done heretofore 

 toward determining the composition of their tests or skeletons. The few published data, more- 

 over, have been generally but not always incomplete in certain particulars, especially with 

 regard to the localities from which the specimens analyzed were obtained, the depth of water 

 in which each creature lived, and the temperature of its habitat. Even in our own work some 

 of these details are lacking, but their great significance when known is strikingly evident in 

 the study of such groups of animals as the echinoderms and alcyonarians. Fm'thermore, 

 some of the older analyses are unsatisfactory for other reasons, for man}- of them were made 

 to determine single facts, such, for instance, as the proportion of magnesium carbonate alone: 

 and in others such essential constituents as organic matter were not taken into account, an 

 omission that is especially serious, for it renders the accurate comparison of the analyses with 

 others impossible. In the shells of mollusks the proportion of organic matter is small, but in 

 the echinoderms, alcyonarians, and phosphatic brachiopods it is relatively large, ranging from 10 

 to even 40 per cent or more. This statement, however, must not be misconstrued; it refers, 

 of course, only to ti;ie specimens, dried or alcoholic, which were actually analyzed. In order to 

 compare the analyses, therefore, so as to determine the true composition of the inorganic shells 

 or skeletons, the very variable amounts of organic matter must be rejected, and the remainders 

 recalculated to 100 per cent. Relations then appear which are not recognizable when the crude 

 unreduced analyses are compared. Any one of oiu- tables of analyses will show this fact very 

 clearl}". 



In a strict sense completeness can not be claimed even for our analyses. Minor constituents 

 which have been detected in marine organisms, such as barium, strontium, fluorine, manganese, 

 copper, zinc, and lead, have been ignored. They occur, as a general rule, onl}- in traces and ha\e 

 little or no significance with respect to the larger problems before us. What organisms tend 

 to form limestones and what ones are notably magnesian, phosphatic, or sihceous are the ques- 

 tions which we are attempting to answer. In most of the analyses lime, magnesia, phosphoric 

 oxide, sulphur trioxide, silica, and loss on ignition were determined. Alumina and oxide of 

 iron were also considered and weighed together. The loss on ignition covered carbon dioxide, 



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