Examination of'-'- Eozoon." 277 



quired : a slide which he had specially prepared for us, con- 

 taining NummuUtes hroachensis infiltrated with mineral 

 matter, is particularly instructive. 



Before offering any opinion on the alleged " explanation," 

 we shall make a few remarks preparatory to its consideration. 



All the investing chambers * of a nummulite are indi- 

 vidualized by walls and a roof'\ — the former being vertical 

 or variously inclined to the plane of the median chambers, and 

 the latter more or less parallel to the same plane. The roofs 

 form an imjwrtant portion of the shell-layers of whicli a num- 

 mulite consists : and the walls, besides bounding the chambers 

 laterally, extend iqnoards, passing through the suprajacent 

 layer. Frequently, a number of walls are piled above each 

 other, and the roof of one chamber serves as the basement 

 of another. The walls are usually thin, but widest at the 

 top ; occasionally thick, as when situated at the junction 

 of three or four chambers : the wall-extensions are often 

 thicker, especially in the latter case. Both are pellucid. They 

 form the " pillars " or " cones " (inverted) previously alluded 

 to. In certain species (probably in all) the walls exhibit an 

 asbestine or fibrous structure, the divisional lines being at 

 right angles to that portion of the layer to which they indi- 

 vidually belong. The roofs (possibly also asbestine) are 

 opaque, of considerable areal extent, and generally thick : they 

 are everywhere penetrated by fine tubules, which lie parallel 

 to the above divisional lines. 



It will thus be understood that every chamber is separated 

 from those adjoining by a vertical asbestine wall, and that every 

 layer is made up of roofs and walls. The layers, consequently, 

 consist largely of tubular, and, to a much less extent, of asbes- 

 tine portions, in alternating order J. Through various pecu-- 

 liarities characteristic of, and irregularities incidental to the, 



* The median chambers require no particular notice, not being directly 

 concerned in the present question ; and, for the same reason, only inci-; 

 dental allusions are made to the " canal-system." 



t Believing that they are more explanatory, we have given these 

 names respectively to the pai'ts usually called septa and wall : the latter 

 is often designated '^nurmmtlme laj/er,'^ &c., from being tubulated, as in 

 the Nummulites. 



I The resemblance of the asbestine to the fibrous structure of arrago- 

 nite is so close as to suggest that, instead of being original, as assu- 

 mable, it may be superinduced and of iuorganic origin, resulting from 

 fossilization. Much could be said in favour of this view : nevertheless, in 

 most cases of a change of the kind stated that have come under our notice, 

 the process has been more or less destructive of original structure, ob- 

 literating the difference between the roof and the walls, or converting 

 alike their substance into a structureless and pellucid condition, often so 

 unequally, that certain lamina? of the shell-layers are opiique, and show 



