10 Prof. W. King on Spirifer cuspidatus. 



on my authority. Dr. Carpenter has not alluded to them j 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 circum- 

 stance will be accepted by any mineralogist as showing that 

 the substance of the former is in a dififerent 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, ^. 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 

 imsatisfactory ; 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. Sc. Thiladelphia, Dec. I860, p. 277. 



