CONCLUDING SUMMAEY: — EXTENT OE EANGE OF VAllIATION. 
671 
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. Caeter) with specimens of the 
Scindian fossil which presents the characters ascribed by M. d’Orbigny to his genus 
Gyclolina, I am now able most fully to confirm the suggestion I threw out on a former 
occasion 49, 70), that this genus is founded on a mere variety of Orhitolites, 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 Helicostegues. For, as I have shown, although Orhitolites 
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 Cyclostegue character would not have been suspected. 
239. Again, I have shown that a parallel variation is displayed by the genus Orhiciilina, 
whose ordinarily helical plan of growth has caused M. d’Oebigny to range it among his 
HMicostegues, 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 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 Orhitolites. 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 CyprcBa 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 Orhiculina has undergone this change, the outer 
* It has been remarked by Messrs. Paekeb and Ettpeet Jones, that whilst the assumption of the 
cyclical form in Orhiculina 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. 
4 F 2 
