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SUPPLEMENT TO THE BRITISH 
being silicified, it became possible by the aid of some diluted acid to gradually dissolve 
and remove the carbonate of lime vi^hich filled the shell or coated its lamellae, so as to 
expose the coils, as was the case in specimens (PI. II, figs. 19, 20, PI. XVII, fig. 12) ; but 
although many thousand examples of Carboniferous Spirifer, Athyris, and Metzia have 
passed through my hands, during the five years I devoted to the preparation of my 
Monograph, it was in very few instances, indeed, that I was able to obtain specimens 
showing the spiral appendages. 
This fact having become apparent to the Rev. Norman Glass, of Manchester, that 
gentleman wrote to me on the 19th of June, 1878, that he had found a process, which 
it had taken him a long time to discover, by which he is able to develop the spiral 
appendages in a most complete manner. 
In a large number of specimens the shell, instead of being filled with a hard lime- 
stone matrix, is partly or wholly filled with spar, and it is to such specimens alone that 
the process which Mr. Glass has discovered applies. There are two advantages con- 
nected with the spires surrounded with spar . — Pirst, though the spar is sometimes very 
hard, yet it is easier to work than the still harder limestone. Indeed, in this latter 
material it is hardly possible to show anything of the spires save by sections, whereas, 
in the spar, the surface of the spires and every undulation and peculiarity of shape can 
be revealed by scraping away the matrix. Secondly, even if the surface of the spires 
could be similarly uncovered in the hard limestone, yet the result would be very 
unsatisfactory, because the spiral lamellae would be so nearly of the colour of the matrix 
as to make it difficult in many cases to see them distinctly, whereas, by proper working 
of the sparry matrix, the spires may be revealed in most beautiful relief, the dark lines 
being clearly traceable in the white surface of the spar. The Rev. Norman Glass 
informs me that in proceeding with any specimens possessing the required conditions 
it is first needful to remove one valve by filing down its inequalities and subjecting the 
smoothed surface to the action of hydrochloric acid. After this great care must be 
taken in working down the spires with a knife of hardened steel, and with water fre- 
quently applied. When every trace and film of the sparry matrix has been removed 
from the spires, the surface must be smoothed with emery cloth, and then for a few 
seconds dipped in the acid. This last and very brief application of the acid takes away 
the dulness of the surface, and gives to it a glossy and transparent appearance as if it 
incorrectly draws as if round, being regularly coated with crystalline carbonate of lime, so that he quite 
misunderstood the nature and character of the spiral coils. AVe now and then meet with specimens in 
this condition, and in which the lamina forming the spine (as in Sowerbj's specimens) has been 
completely dissolved, leaving only a small empty slit surrounded by crystallised carbonate of lime. 
Mr. John Aitken, of Manchester, has recently picked up a beautiful pair of spiral coils surrounded or 
coated with crystalline carbonate of lime, which had been weathered out and detached from the interior 
of a Spirifer, and showing its form in a remarkable manner (Sup., PL XXX, fig. 18). This curious 
specimen, and one or two other examples which I have seen, were obtained from Carboniferous Limestone 
deposits at Twiston, five miles north-east of Clitheroe in Lancashire. 
