Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/101

1872.] FISHER — CRETACEOUS PHOSPHATIC NODULES. 59 fossils may be appreciated. The recent Alcyonium attaches itself to oyster-shells, or other bodies, by a disk, which retains the impress of the nidus. But it is easily removed from its attachment when dead. I am persuaded that when a perfect nodule is examined, a disk of attachment can often be detected, at one end, in the digit-like forms, as if they had grown upon a shell, and in the amorphous nodules at some other part, giving in this case the appearance of the attachment having been effected to sticks or seaweed. It appears to me that the two varieties of form are quite in accordance with the modes of growth which would result from these two modes of attachment.

Nodules of the second more common type appear to be incrusting forms, whether sponges or Alcyonaria. They appear to have grown upon an axis of sea-weed or of wood; for the internal cavity does not seem to have been a cloaca, because it passes from end to end, and the specimens are not funnel-shaped, the two extremities of the hollow cylinders being similar, and finished off alike with a cushion-like rounded edge, and frequently the cylinder is not complete. Specimens of elongated masses are not uncommon which look, at first sight, like pieces of fossil wood, being composed of a bundle of longitudinal fibres, not ill corresponding with the internal longitudinal canals of Alcyonium digitatum described by Johnston, and figured by him*. These, if Alcyonaria, may be individuals which have become mineralized after the destruction of their integument.

Thus these two forms appear to fulfil the conditions described by Johnston as externally characterizing the genus Alcyonium. "Polypemass lobed, or incrusting, spongious, the skin coriaceous, marked with stellated pores;" to which description is added for A. digitatum "the skin somewhat wrinkled, studded over with stellated pores even with the surface†. The "interior of the Alcyonium is gelatinous, netted with tubular fibres, and perforated with longitudinal canals terminating in the polyp-cells, which are subcutaneous and scattered." In the A. digitatum‡ "the space between the tubes is occupied by loose fibrous network," and "the interstices of the whole are filled with transparent gelatine, in which numerous crystalline irregular spicula lie immersed." These spicula are calcareous.

An examination of sections of these nodules does not assist us much in their determination. They are highly mineralized; for they exhibit wide shrinkage-cracks. And of the specimens I have had cut, those which promised to be most sound from the integrity of the outer surface, have turned out most cracked inside. This is exactly what happens in the case of ordinary septaria.

Where so much contraction has occurred, amounting to perhaps one third of the area, of the section, it is not surprising that delicate structure should have been obliterated. The features that can be made out appear to me to be the following:—

The general mass is pervaded in many parts by a minute shading


 * Johnston's 'Zoophytes,' 2nd edition, vol. i. p. 176.

† Ibid. p. 176.

‡ Ibid, p. 176.