1872.] FISHES — CKETACEOTTS PHOSPHATIC NODULES. 55 



phous lumps, varying from a very small size to pieces weighing about 

 a quarter of a pound, though few are so large as that. The other 

 variety, which, though numerous, is less so than the former, consists 

 of long pieces, of such a form as would be produced by taking a cake 

 of dough of the shape of an elongated ellipse, and folding it over a 

 stick laid upon its longer axis. Sometimes the edges are closed, but 

 often not so, and sometimes only partially. Frequently they appear 

 to have been originally closed, but afterwards cracked open, owing to 

 the contraction Caused by mineralization. The hollow axis of this 

 variety is occupied by indurated marl with grains of chlorite. These 

 elongated subcylindrical lumps attain a larger size than the other 

 kind, often occurring of six ounces weight or more. 



These two forms are alluded to by Mr. Seeley in his paper on the 

 rock of the Cambridge Greensand*. He supposes them to have 

 been formed from gelatinous phosphate of lime derived from seaweeds 

 and decaying animals, and rolled on the shore into nodules, with 

 carbonate of lime and the materials of the strand. 



Mr. Bonney, in a paper read before the Geologists' Association f , 

 considered the nodules to be mainly of concretionary origin ; for they 

 are too pure to be regarded as clay saturated with phosphate. 



Upon examining a collection of the more unusual forms of the 

 nodules, other very distinct varieties may be distinguished. It was 

 when examining a set of these in the Woodwardian Museum more 

 than a year ago that I first formed the opinion, from their arranging 

 themselves so decidedly under what seemed to be species, that I had 

 before me true fossil forms. There was one very perplexing specimen, 

 of a cylindrical shape, and marked on the surface somewhat after the 

 manner of the scales of a fir-cone. Mr. Walter Keeping told me that 

 Mr. Jesson, of Trinity College, had a specimen of this, which showed 

 it to be the interior of the fossil known to him as Scyphia tessellata. 

 And upon examining Mr. Jesson's specimen I saw that this was un- 

 doubtedly the case. I was so far confirmed in my opinion. 



I have had thin sections made of a few recognized organic forms, 

 and also of the ordinary nodules, both amorphous and cylindrical. 



The same general character of the matter of which they are com- 

 posed prevails in all ; and the manner of their petrifaction is alike. 

 When seen by transmitted light they are of a more or less bright 

 amber colour, or light brown ; arid when placed beneath the micro- 

 scope appear minutely shaded with dark ramose lines, and speckled 

 here and there with straight or very slightly curved spieular rods, 

 sometimes with sides parallel, sometimes like the acute accent used 

 in printing, and occasionally pointed at both ends. Although the 

 surface of the nodules has a porous appearance, I have not been able 

 to trace any canals connected with the seeming pores. They are 

 traversed by shrinkage-cracks, which appear to reveal facts of im- 

 portance towards interpreting other appearances. 



When such a crack commences at the surface it is usually void 

 for a small space, the soft matrix which occupied it having been 

 removed in the preparation of the specimen. A little further in, it 



* Geol. Mag. vol. iii. p. 305. t See abstract in Geol. Mag. vol. is. p. 144. 



