183 
Possessing only one specimen, which can faiily be referred to the 
stones here treated of, my inquiries respecting them are not likely to 
be so successful as I could wish. In this specimen alone, however, I 
am convinced that I have discovered marks sufficiently strong to prove 
that the coat by which these entrochi are united was extended over 
them during the life of the animal, to produce some effect essential 
to its (economy. 
One of these entrochi (a) passes upwards in a straight line, and is 
nearly surrounded by a spiral turn of the second (b); both being nearly 
invested by a superinduced spathose matter, marked by two seams 
running in the line of contact of the two entrochi. 
In the lower and fore part of this specimen the twisted column 
is less thickly covered by this additional matter, and the straight 
column is quite free from it, for^a small space. It is to the exa- 
mination, which is thus fortunately admitted, of this small part, that 
I am indebted for the ability to add one more conjecture, as to the 
nature and use of this substance by which the two series of trochitae 
are united together. Part of the trochitae of the straight column are 
thus brought to view ; and these are, at first sight, seen to differ in their 
colour from trochitae in general : the discolouration being very different 
from that which is found to proceed from different impregnations 
of a mineral nature. The colour which is here observable is a livid 
grey, such as would not demand any long consideration in one ac- 
customed to see diseased bone, to induce him to suspect, from the colour 
alone, that these parts had been under the influence of disease. 
Being thus impressed by this appearance, I anxiously sought for evi- 
dence which might determine how ill or well this suspicion was 
founded ; and therefore proceeded to make inquiry, whether such con- 
comitant circumstances existed here, as might be expected, if any 
analogy existed between the effects of the assumed disease of these 
parts and those which are known to accompany the disease of the 
bone. 
