GENUS ORBITOLITES -.—GENERAL PLAN; COMPLEX TYPE. 
201 
SO that the height of the columnar segments progressively increases, and the entire 
disk becomes somewhat biconcave. Sometimes, again, without any alteration in 
the thickness of the several parts, the disk comes to assume, by the depression of its 
central portion, the shape of a plate, or that of a watch-glass, or (by the more com- 
plete upturning of its edges) that of a saucer. In any case in which either surface 
of the marginal zone is more exposed by its projection than those of the zones whicli 
it encloses, there will be a special liability to a laying-open of its cells (as shown in 
Plate VII. figs. 8, 10) if the disk should be subjected to attrition; and I believe that 
not only the recent species O. marginalis, but the fossil O. rnacropora, are nothing 
else than examples of this type, the figure of the latter given by Goldfuss * corre- 
sponding exactly with a form of it which I have frequently encountered. I have not 
met with any examples in this simple type, of that marginal thinning away as age 
increases, which is observable in many other Foraminifera. 
24. From the simplest, it will be convenient to pass at once to the most complex 
type of structure presented by the Orbitolite, the existence of which is marked (as 
already noticed, ^ 13) by a multiplication of the horizontal ranges of marginal pores. 
I have met with this form in specimens obtained by dredging, from the coast of 
Australia, from various parts of the Polynesian Archipelago, from the neighbourhood 
of the Philippine Islands, from the Red Sea, and from the idigean ; and as the sands 
of all these localities present the simpler type in great abundance, I am disposed to 
believe that the former is really not the less widely diffused than the latter, and would 
be discovered wherever it abounds, if properly searched for. The largest specimen 
in my possession, measuring seven-tenths of an inch in diameter, is from the coast of 
Australia, where these Orbitolites are so abundant at certain spots (as I learn from 
Mr. Jukes), that their entire disks and fragments, with fragments of Corallines 
(chiefly, I believe, the Corallina palmata of Ellis), constitute the great mass of the 
dredgings. Among the Australian specimens, several attain a diameter of -45 inch, 
and a considerable proportion as much as *30 of an inch. Hence the Orbitolites of 
this type are among the largest forms of existing Foraminifera, being only surpassed, 
as far as I am aware, by the Cycloclypeus hereafter to be described. Of two speci- 
mens in my possession from the Feejee Islands, one measures '63 inch, and the 
other -53 inch in diameter; but the average of the Polynesian specimens, like that 
of the Philippine, Red Sea, and ^Egean, seems to be considerably lower than that of 
the Australian, as their diameter seldom exceeds -25 of an inch, and is usually not 
more than -10 or *12. 
25. The disks formed on this plan, like the preceding, may be considered as typi- 
cally circular, although they are seldom or never exactly so in reality. They may 
be considered, too, as typically flat, with a slight concavity in the central part, from 
which, however, the nucleus often projects; but, as will hereafter appear, there is no 
constant relation either between the thickness and the diameter of different specimens, 
* Petrefacta, pi. 12. fig. 8. 
