196 PEor. V,'. J. soLLAS ox saccammina caeteet, [vol. Ixxvii, 



We have now to consider the systematic position of the fossih 

 The complete mineralization which it has undergone seemed to 

 render it doubtful ^vhether the existing structure of the wall could 

 be considered original, or whether it might not be due to a j^ost- 

 moj'tem molecular rearrangement ; and this led me to doubt whether 

 it was ever arenaceous, and consequently whether it had any claim 

 to a close alliance with Saccammina spliarica, such as is generally 

 assumed. 



It became necessary, therefore, to search in the first place for 

 some criterion by wliich the perforate and imperforate foraminifera 

 mig^ht be distino-uished one from the other when occurrins: in the 

 fossil state. I was thus led to investigate the minute structure of 

 their shell, and succeeded in obtaining some results, which, though 

 by no means exhaustive, seem to be of sufficient interest to be 

 introduced here. As a preliminary, I ought to remark at once 

 that there is no difference in the mineral composition of the two 

 kinds of shell : it consists both in Perforata and Imperforata of 

 calcite. The statement, for which I am responsible, that the shell 

 of the Imperforata consists of aragonite is erroneous ; it was based 

 on determinations of specific gravity, which, though correct in 

 themselves, were made on specimens containing foreign mattei'. the 

 presence of which was not suspected at the time. 



Dr. J. J. Lister was the first to establish the true nature of the 

 mineral present in the Imperforate shell ; he found that the shells 

 of both Perforata and Imperforata give the well-known calcite 

 reaction with cobalt nitrate. I have repeated his experiments Avith 

 the same result, and have obtained further confirmation by treat- 

 ment with ammonium ferrous sulphate. As these tests, however, 

 are not always decisive,^ renewed observations were made on the 

 specific gravity of the shells both of the Perforata and Imj^erforata, 

 using for the former some Glohigerina ooze, rich in Orhulina, 

 which w^as dredged bv the ' Challenger ' from 1990 fathoms in 

 lat. 20^ 17' S., long, li^ 2' ^\ . This was freed from fine powder 

 by washing in water, dried, and placed in a diffusion-column. The 

 Glohigerina and Orhulina gave results of no value, owing to 

 the presence of impurity, probably argillaceous matter ; hut on 

 the Orhulince being crushed swarms of young Glohigerina were 

 set free ; they were clear, colourless, and transparent, and floated in 

 a zone of specific gravity ranging from 2-711 to 2"706. 



For the Imperforata, Orhifolites from the sands of Funafuti 

 were selected. After being dried at 100" C. they were ground to a 

 fairly fine powder in an agate mortar, and placed in a di:ffiusion- 

 column where they formed a dense zone of mean specific gravity 

 2"72-l ; but many fine particles and some coarser fragments 

 extended upwards to 2'65, and a smaller quantity downwards to 

 2 "86 or more. 



If we take the sjDecific gravity of the zone as a basis, and assume 

 the presence of organic matter to the extent of I'-l per cent., as in 



^ J. Johnston, H. E. Merwin, & E. D. "Williamson, ' The Several Forms of 

 Calcium Carbonate ' Amer. Journ. Sci. ser. 4, vol. xli (1916) p. 478. 



