56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, 



will be found to be filled either with clear amber-coloured phosphate 

 or else (less commonly) with glauconite. But towards the interior it 

 ramifies into many minute fissures, which appear black, and, losing 

 themselves and reappearing in parallel lines with cross cracks con- 

 necting them, produce altogether a ramose structure, whose twigs are 

 generally directed in the course of the crack. 



I have noticed that, bordering their course, the phosphate is clearer 

 and more free from the ramose shading than it is at a distance from 

 them, and that the finer ramifications of the cracks assume very 

 much the character, on a larger scale, of that shading. This has 

 led me to doubt whether the shading in question may not be, at any 

 rate partly, due to a physical cause, and not to organic structure. 



These three facts, however, appear to come out clearly from the 

 study of the cracks : — first, that phosphate continued to be de- 

 posited after the mass was already to some extent mineralized and 

 had begun to contract from that cause, and that slightly open spaces 

 became thus infiltrated by it ; secondly, that glauconite was some- 

 times, but more rarely, deposited in a similar manner and in like 

 situations, chiefly by open communication from without ; and, thirdly, 

 that the more minute vacuities within the mass were generally 

 filled by a dark mineral, which I presume to be ferruginous. 



"When the nodules are dissolved in hydrochloric acid, a small resi- 

 duum is left which does not commonly reveal any organic structure in 

 the shape of siliceous spicula or otherwise. In the indurated matrix 

 which fills the hollow axis of the cylindrical nodules a few spicula, 

 apparently siliceous, may be here and there detected, along with the 

 usual grains of glauconite. 



In order to pass from the known, or partly known, to the un- 

 known, I will describe the appearance of a few recognized organic 

 forms, as seen in section under the microscope, before speaking of 

 the ordinary nodules, the nature of which is more obscure. 



The recognized organic forms, three in number, which I have 

 examined have passed by the names of Porospoyigia and Scyphia. 

 They appear to be all Ventriculites, and reveal very plainly the qua- 

 drate, reticular structure described and figured by Toulmin Smith *. 

 And since this structure is seen equally well whether the section is 

 horizontal or vertical, it affords a strong presumption that the true 

 " octahedral " structure of that author is present. 



An examination of these specimens, however, has led me to a 

 somewhat different conclusion as to the nature of the fibres disposed 

 in this quadrate arrangement from that arrived at by Mr. Smith. 

 And this need not surprise one, because the state of preservation of 

 the fossils is entirely different in the two cases. The most instruc- 

 tive of Mr. Smith's specimens, which may be seen in the British 

 Museum, consist of Ventriculites preserved in chalk, of which the 

 organic structure is mineralized by iron oxide. Some of these have 

 been dissected out by the action of weak acid, and show beautifully 

 the quadrate arrangement in minute strings of that mineral. 



The bars, however, which compose the quadrate figures in our 



* 'Annals & Magazine of Natural History', 1st series, vol. xx. pi. vii. fig. 9. 



