CONCLUDING SUMMARY: EXTENT OF EANGE OF VARIATION. 571 



able, the one from the other, without the least difficulty, yet they are often combined in 

 the same individuals, and this in such a variety of modes, that the transition from the 

 simple to the complex may be clearly seen, from the comparison of a sufficient number 

 of specimens, to be by no means attributable to a mere advance of age. Further, 

 having been furnished (by the kindness of Mr. H. J. CARTER) with specimens of the 

 Scindian fossil which presents the characters ascribed by M. D'ORBIGNY to his genus 

 Cydolina, I am now able most fully to confirm the suggestion I threw out on a former 

 occasion (*f[^[ 49, 70), that this genus is founded on a mere variety of Orbitolites, in 

 which the character of the surface-marking is more than ordinarily cyclical. 



238. Not merely, however, does the range of variation of this type confound the 

 ordinary distinctions of systematists in regard to species and genera; it extends also to 

 that difference in plan of growth, which has been assumed by M. D'ORBIGNY to be of 

 such fundamental importance as justly to constitute the essential difference between his 

 two orders Cyclostegues and Helicostegucs. For, as I have shown, although Orlitolites 

 is typically cyclical from its commencement, yet specimens frequently present themselves 

 in which its early development has taken place so completely on the helical plan, that 

 if such had been collected before their assumption of the cyclical mode of growth, their 

 essentially Cyclostcc/ue character would not have been suspected. 



239. Again, I have shown that a parallel variation is displayed by the genus Orbiculina, 

 whose ordinarily helical plan of growth has caused M. D'ORBIGNY to range it among his 

 Helicostegues, notwithstanding that in fully-developed specimens its mode of growth is 

 not unfrequently cyclical. The occasional exchange, in this type, of one plan of increase 

 for the other, at an advanced period of life*, of which exchange I think I have given 

 adequate evidence (f[^f 85-87), is a fact which seems to me of very high interest, being 

 much more decided in its nature than the corresponding change in Orlitolites. For 

 whilst in the latter the tendency of the spiral form (whenever it presents itself) to pass 

 into the cyclical, is apparent almost from the beginning, and the change is never long 

 postponed, the helical plan is that on which the growth of the former not only com- 

 mences, but continues to be carried on, often through the entire period of its increase. 



240. It is important to remark that in each case the metamorphosis is simply due to 

 the very rapid opening-out of the mouth of the spire, its two lateral extremities 

 extending themselves round the shell on the one side and on the other, until they meet 

 and completely enclose the portion previously formed (just as the lobes of the mantle in 

 the adult Cyprcea spread themselves round the shell until they meet on its dorsum) ; 

 and also that the mutual relations of the chambers of the shell and of the segments of 

 the animal body which they contain, remain essentially unchanged. Again, it is a point 

 of no mean significance, that when an Orbiculina has undergone this change, the outer 



* It has been remarked by Messrs. PAEKEB and BTJPEBT JONES, that whilst the assumption of the 

 cyclical form in Orbiculina may often be the result of the continued growth of individuals under favourable 

 circumstances, small starved forms also frequently take on the cyclical condition, leaving the young sub- 

 lenticular stage without passing through the aduncal. See Annals of Natural History, March 1860, p. 181. 



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