GENUS ORBITOLITES: REPRODUCTION; VARIATIONS IN SIZE. 213 



phenomenon of their development, instead of being the result of accident. I do not 

 wish to attach any weight to the interpretations I have here offered ; but I simply state 

 the facts, and the explanations of them which have suggested themselves to my own 

 mind ; merely adding, what I hope to present in more detail at a future opportunity, 

 that bodies resembling the first or primordial cell, in which Foraminifera of all forms 

 originate, are not unfrequently met-with in the chambers of many other species. 



IV. Variations. 



44. Variations in Size. We have already seen that diversities both in the diameter 

 and in the thickness of the disk, arise directly from the degree in which the animal 

 substance (whereon the skeleton is modelled) has extended itself either horizontally 

 or vertically, so as to multiply either the number of concentric rings, or the number 

 of the superposed segments of which each ring consists. This, however, is not the 

 only source of variation in size ; for a most extraordinary diversity presents itself in 

 the dimensions of the individual components, by whose repetition the entire disk is 

 made up. It is in the nucleus that I find this diversity most strongly marked, as 

 will appear from a comparison of Plate VII. figs. 1 4, which exhibit parts of a grada- 

 tional series of twelve, from the smallest to the largest forms I have met with, all of 

 them accurately drawn, under the same magnifying power, from specimens in my 

 possession*. The length of the entire nucleus of fig. 4 is about seven times that of 

 the nucleus of fig. 1, and its breadth aboutybwr times as great ; the area of the former 

 is therefore about twenty-eight times that of the latter ; and as it is also several times 

 as thick, the whole of the cavity, which was occupied in the living state by animal 

 substance, could scarcely have been less than a hundred times as large in the one as 

 in the other. (Compare also figs. 5, 6, 10, 12, 13 of Plate IV.) There is not by 

 any means the same amount of difference between the dimensions of the ordinary 

 cells which are formed by concentric extensions of the nucleus ; nevertheless, it will 

 be seen by a glance at the figures just referred-to, that these also exhibit marked 

 diversities in size, the largest cells being usually found to spring from the largest 

 nuclei, and vice versa. Moreover, the individual cells of the very same disk are 

 occasionally found to differ amongst each other, as widely as do the cells of fig. 1 

 from those of fig. 4. 



45. Similar differences present themselves in the thickness of individual cells ; as is 

 of course best seen in the simple type of Orbitolite, in which the augmentation of 

 thickness is not produced by the vertical superposition of multiple segments. A 

 remarkable example of this kind is presented in the comparison of figs. 4 and 5 of 

 Plate V. ; these being, like the figures in Plate VII., drawn under the same magni- 

 fying power. I possess a series of vertical sections of different individuals, in which 

 the same gradual transition is seen from the thin to the thick, as I have just stated 



* The entire series of figures is in the possession of the Royal Society. 

 MDCCCLVI. 2 F 



