GENUS ORBITOLITES. 113 



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 "-^S 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 Cyclochjpeus hereafter to be described. Of two 

 specimens in my possession from the Fiji Islands, one measures '63 inch, and the other -53 

 inch in diameter ; but the average of the Polynesian specimens 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. 



1 64. The disks formed on this plan, like the preceding, may be considered as typically 

 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 

 primitive disk often projects ; but, as will hereafter appear, there is no constant relation either 

 between the thickness and the diameter of different specimens, or between the thickness of 

 different parts of the same specimen and the distance of these parts from its centre. The 

 only remarkable departure from the ordinary form which I have met with, presents itself in 

 certain Orbitolites from the Fiji Islands, of which several specimens in the Museum of the 

 Royal College of Surgeons, and two in my own possession, exhibit a curious plication towards 

 their margins ; the degree of this departure varies so much, however, in different individual? 

 (the plication being almost obsolete in some), that it cannot be admitted to mark a specific 

 diversity. These same specimens, moreover, also exhibit another curious abnormality ■ 

 namely, the projection of the upper and lower edges of the margin, so that a groove is left 

 between them, the projecting laminae being thin and fohaceous, and their chamberlets very 

 irregularly arranged. This peculiarity, again, being far from uniform in its degree, and being 

 altogether wanting in specimens which in other respects precisely resemble those with plicated 

 and foliated margins, must be considered merely in the light of an accidental variety ; but I 

 cannot suggest any explanation of its occurrence, or of its limitation (so far as I am aware) to 

 this particular locality. 



165. The surface of the disk (Plate IX, fig. 7)* is marked out, as in the simpler type, by 

 concentric zones, the number of which bears a general (though not a constant) ratio to its 

 diameter; these zones are traversed by radiating lines, which mark out areolae that are 

 usually somewhat rectangular in shape and sometimes approach a square, but are more com- 

 monly nearly twice as long in the line of the radius of the disk as they are in the transverse 

 direction, their long sides being nearly parallel to each other. We shall hereafter see, however, 

 that the form of these areolae is very subject to variation, and that it may be very dissimilar' 



* In order to avoid a multiplication of figures, I have thought it preferable to combine in three 

 ideal representations (Plate IX, figs. 7, 8, 9,) the details I have made out from a great number of 

 preparations which are faithfully represented in separate figures accompanying the original Memoir in 

 the Arcliives of the Royal Society ; these last of course furnish the real authority for every point in the 

 description, the ideal figures, however, serving to display the relation of different parts to each other 

 in a manner that no single preparation would possibly admit. 



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