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



dial segment of the other gives off a far thicker cord, which only makes a single turn. It can 

 scarcely be doubted, I think, that this circumambient segment represents the whole of the 

 original "spiroloculine" coil drawn up into itself, and thus perpetuates, under a form which 

 at first sight appears entii'ely unrelated, the " milioline " plan of origin ; thereby giving 

 the key to the import of this " nucleus " in the more specialised forms to which we shall 

 next proceed. 



From this point of view, it is a circumstance by no means insignificant, that even the 

 varietal forms of this well-marked species present a gradational transition to the next, in 

 the diminished excentricity of the " nucleus," the less marked restriction of outgrowi^h to 

 one side of it, and the consequent earlier exchange of the spiral for the cyclical plan of 

 growth. 



Geographical and Bathymetncal Distribution. — This species appears to be pretty 

 generally diffused along the littoral zone of the warmer temperate and tropical seas, 

 being met with abundantly in shore-sands and in shallow-w\ater dredgings. It seems 

 least common, however, in West Indian seas, where it is replaced by the small varieties 

 of Orhiculina adunca. The largest specimens of it hitherto obtained are those brought 

 up in the 18 fathoms' dredging of the Challenger on the Fiji reef. So far ^s can be judged 

 from the specimens contained in shore-sands, this type attains a much smaller size in 

 the Eed Sea, although numerically abundant. And it would seem to die out in the 

 Mediterranean and ^gean, where it is a comparatively rare form, and stunted in its growth. 

 Hence its most congenial habitat may be said to be the littoral zone of tropical or sub- 

 tropical seas. — It is worthy of note that the small Red Sea disks often have their sirrface- 

 layers thickened by an irregular exogenous deposit of shell-substance, which obscures the 

 cyclical arrangement that i~ so conspicuous in the large Fijian specimens. The somewhat 

 larger disks of Philippine and Australian shores often exhibit irregular radiations of 

 such deposit ; but between these radiations the cyclical arrangement is generally 

 conspicuous. 



Geological Distribution. — Among the Orbitolites that have been described as fossil 

 there does not seem any that is distinctly referable to this t}^e. I am inclined to think, 

 however, that the CycloUna armorica of d'Archiac, the Archiacina armorica of M. 

 Munier-Chalmas (to whose kindness I am indebted for specimens of it), may be regarded 

 as an ancestral form of Orbitolites mai-ginalis. Though the diameter of its disk does 

 not exceed that of the largest specimens of Orbitolites marginalis, its thickness is two or 

 three times greater ; this excess being partly due to the thickness of the superficial shell- 

 deposits, and partly to that of the chambered layer they enclose. The cavitary system 

 appears, in its earlier stage, to have been distinctly " peneropline," without division of the 

 chambers into chamber! ets; and to have early become "cyclical "by the extension of 



