GENUS ORB1TOLITES : 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 which 

 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. macropora, 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 ^Egean ; 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 yEgean, 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. 



