10 Prof. W. King on Spirifer cuspidatus. 
on my authority. Dr. Carpenter has not alluded to them ; but 
they are attested by Mr. Meek, who states that “where the 
perforations happen, as is often the case, to be filled with 
matter of the same colour and translucency as the fibres com- 
posing the shell, it is exceedingly difficult to see them”’*. 
It is now necessary to refer more particularly to a point 
already mentioned, viz. that the imperforate specimens have 
a higher degree of hardness than the perforated. The cireum- 
stance will be accepted by any mineralogist as showing that 
the substance of the former is in a different condition from that 
of the latter. In both cases, however, the substance is carbo- 
nate of lime: it may therefore be concluded that the softest or 
perforated specimens are composed of this compound in its 
ordinary state, 7. e. calcite, and the hardest or imperforate ones 
in the dimorphic state of arragonite. Still the question requires 
to be answered—why is it that the fibrous tissue is preserved, 
and that the perforations are obliterated ? 
In the course of my present investigations I have repeatedly 
traced the perforations passing by degrees into obscureness— 
their opacity insensibly melting into a translucency approach- 
ing that of the fibres, and their indefinite outline gradually 
becoming still more indefinite. Between either of the last 
states and total obscurity on the one hand, or complete inde- 
finiteness on the other, I have not been able to trace the per- 
forations with any satisfaction. But numerous translucent 
spots, large and small, may often be observed. It is the num- 
ber of these spots that makes the investigation at this stage so 
unsatisfactory ; nevertheless there is nothing to oppose the 
idea that the largest of them represent the perforations, and 
that the smallest are cross sections of individual fibres, or 
bundles of them, which have curved off from the general plane 
to which they belong. 
Moreover in perforated specimens the fibres are composed 
of an amorphous translucent substance, and the contents of the 
perforations of granular opaque matter, both structures most 
probably consisting of carbonate of lime. The reason of the 
perforations being obvious in such specimens requires no ex- 
planation. Assuming, however, the existence of specimens 
with both the fibres and the contents of the perforations 
changed into amorphous translucent arragonite, is not such a 
condition the very one to render it impossible for the perfora- 
tions (so small and indefinite as they are in Spirifer cuspidatus) 
to be distinguished from the fibres ? 
But, whatever way the question under consideration is to be 
* Proc, Acad. Nat. Se. Philadelphia, Dec. 1865, p. 277, 
