1872.] ITSHER — CRETACEOUS PHOSPHATIC NODULES. 59 



fossils may be appreciated. The recent Ahyonium 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, a,t 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 com- 

 plete. 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 inte- 

 gument. 



Thus these two forms appear to fulfil the conditions described by 

 Johnston as externally characterizing the genus Alcyonium. " Polype- 

 mass lobed, or incrusting, spongious, the skin coriaceous, marked 

 with stellated pores;" to which description is added for A. digi- 

 tatum " the skin somewhat wrinkled, studded over with stellated 

 pores even with the surface "f. The "interior of the Alcyonium 

 is gelatinous, netted with tubular fibres, and perforated with longi- 

 tudinal canals terminating in the polyp-cells, which are subcuta- 

 neous 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 nume- 

 rous 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. 17C>. 

 t Ibid. p. 174. + Ibid, p. 176. 



