CONCLUDING- SUMMAEY: EXTENT OP 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 differentiation may be thus expressed : 



Heterostegine type 



diverging into 



Operculina Heterostegina 



A. 



~ 



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 OrMtoides 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 Rotalia 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 



