66 Cornish and Kendall—Calcareous Organisms. 
IV.—On roe MineERALoGicaL ConstituTION OF CALCAREOUS 
OrGANISMs.? 
By Vaueuan Cornisu and Percy F. Kenpatt, 
Berkeley Fellow of the Owens College. 
Introduction. 
a. Mr. Sorby’s presidential address at the anniversary meeting of 
the Geological Society in 1879, attention was drawn to the fact 
that the carbonate of lime in calcareous organisms is in certain cases 
in the form of calcite, in others of aragonite, and various genera of 
such organisms were classed according to their mineralogical con- 
stitution. It was also shown that aragonite fossils are of greatly 
inferior stability to those formed of calcite, in many deposits casts 
only of aragonite fossils being preserved, whilst those of calcite 
remain unaltered. In the same address Mr. Sorby insisted on the 
importance of this difference of stability as affecting the trustworthi- 
ness of the geological record. 
The idea that the disappearance of aragonite fossils is due to the 
action of carbonated water naturally suggests itself; at the same time 
no experimental data appeared to exist which would lead one to 
suppose that calcite would be acted upon less readily than aragonite 
by a solution of carbonic acid. 
Part I. of the present paper contains an account of the experimental 
evidence obtained as to the cause of the inferior stability of aragonite 
fossils as compared with those formed of calcite, with observations on 
the geoloyical conditions favourable to the removal of aragonite fossils. 
It was pointed out by one of us in a paper in the GroLocicaL 
Magazine, Nov. 1883, that those shells classed by Mr. Sorby as cal- 
cite are characterized in the fossil state by a compact texture and by 
translucency, whilst the aragonite shells are opaque and of a chalky 
appearance, and the opinion was there stated that these characters 
would be found sufficiently constant to be of use in determining the 
zoological position of obscure forms. 
Part II. of the present paper contains an account of the work done 
in following out the above observation, and in the examination of 
certain organisms belonging to groups not yet classified according to 
their mineralogical constitution. 
Part J. 
Two fossil shells, Pecten opercularis (calcite) and Pectunculus 
glycimeris (aragonite) were selected, not differing greatly in weight, 
and presenting nearly the same extent of surface. The aragonite 
shell was a specimen entirely unacted on, with a hard compact 
surface. These shells were suspended in a solution of carbonic 
acid, removed. from time to time, weighed, and then placed in a 
fresh solution of carbonic acid. The experiment was only dis- 
continued when the aragonite shell fell into fragments. 
1 An Abstract of this paper was read before Section C. of the British Association 
at Manchester, September, 1887. 
