454 NATURE AND MAN. 



4), constituting but a single plane ; those of each series being 

 connected together by a single continuous annular gallery (shown 

 in cross-section at (ac, ac), while those of each series are connected 

 with those of the next by single radial passages (r, r, r), which, as 

 each annulus was formed, would open at its outer edge as a single 

 row of marginal pores. Bat these are surrounded by rings {d, d}, 

 cP) in which, while the annular canal is still single, two radial 

 passages (r) go off from it obliquely, one into the upper and the 

 other into the lower portion of each chamberlet of the next 

 annular series, those of the last-formed annulus showing them- 

 selves at its edge as a double row of marginal pores. From this 

 "duplex" type, the first advance towards the "complex" is shown 

 at e, e^, in the splitting, so to speak, of each annular canal into 

 two {ac, ac'\ and the interposition of a columnar cavity {m, in) 

 between its two halves. Now, in the inner (or earlier-formed) of 

 the annuli which show this complication {e^ e^), the two series of 

 chamberlets {s s, s' s') which lie between the two annular canals 

 and the two surfaces of the disk, are continuous with the inter- 

 mediate columnar chamberlets, and bear the same relation to their 

 respective annular canals as in the " duplex " type, each being 

 connected with one canal only ; and this stage of differentiation 

 characterizes the Orbitolites of the French Tertiaries, which seem 

 to have attained their full growth without any advance upon it. 

 But in the large Orbitolite disks of Australia and Fiji, I find this 

 simpler arrangement giving place to a more complicated one 

 (/, /I, /^, /^) ; the chamberlets of the two superficial layers being 

 separated from those of the intermediate layer, and being so 

 shifted in position, that each annular series lies over the interval 

 between two annular canals, and communicates with both of them ; 

 while the sarcodic body which occupies this cavitary system thus 

 comes to have the more complicated arrangement shown in 

 Fig. VIII. With the increase in the thickness of the intermediate 

 layer, the double row of marginal pores of the " duplex " type 

 gives place to the multiple series (Fig. VII., ;;//) of the "complex." 

 Now it seems to me impossible not to recognize the fact, that 

 the evolution of this type has taken place along a definite course ; 

 every stage being one oi progress, and each being (so to speak) a 

 preparation for the next. This, perhaps, will be most clearly seen 



