120 FAMILY MILIOLIDA. 



the first place, that there is no relation of necessary dependence between the several portions 

 of the disk ; since not only can the greater part of the peripheral portion be lost without 

 any diminution in the growing power of that which is left, but even a fragment of the peri- 

 pheral portion, altogether detached from the centre, can not only maintain its vitahty, but 

 become the centre of a new disk. Secondly, the growth of the sarcode and the addition of 

 new parts may take place not merely in the peripheral direction from the normal outer margin, 

 but also in the direction of the centre, provided that a free edge be exposed at the inner 

 margin of any zone. Thirdly, the reparative iiisus seems always to tend towards the pro- 

 duction of a disk whose shape shall approach the circular, whatever may be the form of the 

 fragment which serves as its foundation ; thus showing that, notwithstanding the repetition 

 and independence of the separate parts of these organisms, each cluster, whether large or 

 small, is an integer, having an archetypal symmetry to which it tends to conform, — strongly 

 reminding us of the phenomena of crystallization. And fourthly, the plan by which this 

 recurrence to the discoidal form is provided for, seems partly to consist in the limitation of 

 the new growth to the natural margins of the zones ; no such growth taking place from the 

 edge of a fracture which has crossed the zones transversely, although it may proceed from 

 the remains of a zone which has been broken off by a fracture that partly follows its course. 



177. Varieties. — We have already seen that diversities both m \\\e diameter awA 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 tlie length of the columnar segments of which each 

 ring consists. This, however, is not the only source of variation in size ; for a most extra- 

 ordinary diversity presents itself in the dimensions of the individual components by whose 

 repetition the entire disk is made up. It is in the " primitive disk " that I find this diversity 

 most strongly marked, the central area of one specimen in my possession being about 

 twenty-eight times that of another, and every intermediate gradation being presented by other 

 specimens. There is not by any means the same amount of difference between the dimen- 

 sions of the ordinary segments which form the concentric annuli ; nevertheless, these also 

 exhibit marked diversities in size in different specimens (the largest chamberlets being usually 

 found to spring from the largest primordial chambers, and vice versa), and the individual 

 cells of the very same disk being occasionally found to differ no less widely amongst each 

 other. Similar differences present themselves in the vertical height of individual chamber- 

 lets ; as is of course best seen in the Simple type of Orbitulites, in which the augmentation 

 in the thickness of the disk is produced merely by the elongation of the columnar segments. 

 I possess a series of vertical sections of different individuals, in which the same gradual 

 transition is seen from the thinnest to the thickest, as I have just stated to exist in regard 

 to superficial area ; and which also proves that the relative thickness of the central and of 

 the peripheral portions is equally liable to variation. — It seems obvious, from the foregoing 

 considerations, that neither the absolute nor the relative dimensions of the individual parts 

 of these composite fabrics can, any more than the dimensions of the entire disks, be taken 

 as affording valid characters for the discriminatiop of species ; and that such a wide range 

 of variation exists among individuals, as would, if the extreme cases alone were known, seem 

 fully to justify their separation under distinct specific designations. 



