REPORT ON THE GENUS 0RBIT0L1TE8. 41 



Even when it has been completely carried out, the connections of the superficial 

 sub-segments remain exactly what they are in the " duplex " type. For there, as has 

 been shown, each half-column springs from its own sarcodic annulus, and receives at its 

 base a radial stolon from the annulus next interior to it ; the connection between the 

 successive annuli being made by the passage of two series of radial stolons from each 

 annulus (PI. V. fig. 2, d, d, d,'d'), into the two series of half-columns of the annulus 

 exterior to it. And in the " complex " type, as a careful examination of fig. 5 (p. 40) 

 will show, the pedicle by which each superficial sub-segment is connected with the 

 sarcodic annulus lying beneath its obiter extremity (see PI. V. fig. 14) may be considered 

 as its own proper base, whilst that which connects its inner extremity with the annulus- 

 uext interior to it is the homologue of the radial stolon of the " duplex " type. 



Now as the displacement, which at first sight conceals this homology, shows itself in 

 the life-history of certain individuals of the type which are developmentally less- 

 advanced than the rest, it may be pretty safely aSirmed to have taken place in the 

 genetic history of the race. And we have a curious confirmation of this assumption 

 in the fact that the fossil specimens of Orhitolites comj^lanata, which are so 

 abundant in the Paris Tertiaries, show an incompleteness in the process of difi'erentia- 

 tion, which stops at the stage at which the chamberlets of the superficial layers are 

 still continuous with the cylindrical chamberlets of the intermediate stratum. 



If then we were able to trace out the entire Palseontological history of the Orbitoline 

 type, we should pretty certainly find a long succession of intermediate forms, grada- 

 tionally leading up from the " simplest " to the most " complex " ; the typical Orhitolites 

 comiilanata of the present time being the most highly specialised form of it with which 

 we are acquainted. But although its descent from some " simple " form can scarcely 

 be doubted, yet we cannot fairly assume that either of the species previously described 

 represents its ancestral type, and is capable of evolving itself under favourable conditions 

 into the "complex" form. For I not only find a very constant limitation of size to 

 prevail, alike in Orhitolites marginalis and in Orhitolites duplex, of each of which forms 

 I have examined many hundred specimens ; but I have met, in several of the largest 

 examples of each, with that undivided or imperfectly partitioned condition of the 

 peripheral annuli, which seems to indicate the feebleness (so to speak) of old age, 

 rather than such an excess of vigour as would be needed to carry them on to a higher 

 grade. It appears to me, therefore, that the two species just named are to be considered 

 as perpetuating earlier types of the genus ; whilst the occasional occurrence of the 

 " simple " plan in the central portion of the disks of Orhitolites complanata marks a 

 reversion to that earlier plan, which indicates a want of developmental power in the 

 individuals presenting it. And a clue to this deficiency is, I think, to be found 

 in that remarkable inferiority in the size of the " nuclear mass," which I have 

 already spoken of (p. 38) as a constant feature in these sub-typical forms. 



(ZOOL. CHALL. EXP.— PART XXI. 1883.) X 6 



