42 THE VOYAGE OF H.M.S. CHALLENGER. 



Whether such disks constitute a distinct race, or are merely individuals which have 

 begun life as " starvelings " that do not inherit the characteristic vigour of the species, it 

 ■can scarcely, I think, be doubted that they represent an anc^estral form in which the 

 "simple " OrhitoUtcs was undergoing evolution into the " complex"; the early growth of 

 ■every disk in that stage having probably been simple, as we still find it to be in some. 

 In those, on the other hand, in which the perfected type has fully established itself, the 

 ■earlier " simple " Orbitoline stage drops out, as the Peneroplinc and Orbiculine stages had 

 previously done ; so that the " complex " plan of Orbitoline growth now immediately suc- 

 <:'eeds the ^lilioline, in all those forms in which the primordial segment carries with it the 

 full developmental capacity of its predecessor. It is not a little curious, however, that in 

 the marginal aunuli of even this highest type, a reversion to the undivided Pcneropline 

 <jondition should not unfrequently show itself, in an almost entire want of subdivision 

 •of the annular zones into chamberlets ; the interzonal septa, however, being formed as 

 usual, and being marked by multiple ranges of pores. 



Geographical and Bath ij metrical Distribution.— Ho far as is at present known, 

 Orhitolites coinplanata inhabits only the shallow waters near shores, or on the slopes 

 ■of reefs, in tropical or sub-tropical seas. It has been met with abundantly in 

 such situations on the coast of Australia, on the Fiji and otlu'r reefs in the Pacific 

 Ocean, and in the Philippine Sea; but, notwithstanding the ;il)undauce of Orhitolites 

 marginalis and Orhitolites duplex in the Eed Sea, this most highly developed type 

 has not hitherto l)een found there. As already stated, its largest and most exuberant 

 forms are found in surface-water ; whilst it is among those In'ought up by the dredge 

 from a deeper part of the littoral zone, that those " sub-typical " specimens occur in largest 

 proportion which in the earlier stage of their growth present the " simple " type of 

 formation. 



Geologiccd Distrihutioit. — Xs already stated, the specimens upon which not only tlie 

 species but the genus is constituted belong to the early Tertiary period : tlie Calcaire 

 Grossier of the Paris basin, and corresponding (Middle Eocene) formations elsewhere, 

 containing Orhitolites complanata in such abundance, that the rock in some situations 

 is chiefly composed of its disks. These are often found 0"8 inch in diameter, 

 thus equalling in size all save the very largest of those brought from the Fiji reefs. 

 Orhitolites complanata seems also to occur in the Nummulitic Limestone of the north- 

 west of India; but from the external similarity of its disks to those of Orhitoides, whicli 

 genus also flourished at the same jjeriod, they canu(»t l)e certainly distinguished by 

 the imperfect descriptions of them hitherto given. This difficulty of identification, which 

 applies also to the genus Orhiculina, prevents it from being certainly stated at what 

 Geological period Orhitolites complanata made its first appear-ance. It is reported as 



