CONCLUDING- SUmiAET: — EXTENT OE EANGE OF VAEIATION. 
577 
of the body are much more isolated from each other than they are in the type already 
described ; and the proper walls of the chambers seem, as it were, to be moulded upon 
the segments, instead of merely filling up the interspaces between them. This filling-up, 
in fact, is the office of the “ intermediate skeleton,” which gives a solidity to the whole 
aggregation that would otherwise be wanting ; and special provision, as we have seen, 
is made in the canal-system for its nutrition. Altogether this type is the one in which 
the Foraminiferous structure attains its highest development, and which is most com- 
pletely differentiated from every other. And the morphological variations it is known 
to undergo seem to me fully to justify the inference, that such further variations as 
have been shown to occur in the Orbiculine type might be regarded as the probabl 
source of the divergence, from some common ancestral stock, of the several forms 
whose intimate relationship I have demonstrated. The analogy of that type would 
suggest Heterostegina as presenting the nearest existing approximation to such a 
common original ; and the stages of difierentiation may be thus expressed : — 
Heterostegine type 
diverging into 
; ^ 
Operculina Heterostegina 
. A . . 
. . ; ; ^ . n 
Amphistegina, Nummulites, Operculina. Heterostegina, Cycloclypeus. 
From my imperfect acquaintance with Fusulina, I do not feel justified in expressing its 
exact relationship to either of the forms included in this scheme ; and for the same 
reason I abstain from connecting Orhitoides with Cycloclypeus^ to which it has some 
features of close relationship. 
249. After this detailed examination of the general relations of the principal modifi- 
cations of two of the most strongly marked types to be found in the whole group of 
Foraminifera, it seems needless for me to do more with respect to the other forms whose 
structure I have investigated, than to inquire how far the peculiar characters by which 
they are respectively distinguished show evidence of a like variability. — Thus we have 
seen that Calcarina is essentially distinguished from Motalia by the extraordinary 
development of the “ supplemental skeleton,” and by the extension of this into radiating 
prolongations. But it has been shown 197) that the number, form, and proportions of 
these prolongations are subject to very considerable variations ; so that whilst they are 
sometimes so greatly multiplied and prolonged as to constitute the principal feature of 
the organism, they are so little developed in other instances that the contour of the disk 
is scarcely interrupted by them. Further, it has been shown 207) that the develop- 
ment of this supplemental skeleton is in great degree independent of that of the spire ; 
hence if this last be the essential component of the organism (as all analogy indicates), 
the supplemental skeleton must be regarded as a feature of minor importance. On the 
other hand, the development of radiating outgrowths is an occurrence not unfrequent 
among other helicine Foraminifera, even in species whose typical form is altogether 
MDCCCLX. 4 G 
