572 
DE. CAEPEXTEE’S EESEAECHES OX THE EOEA^nXITEEA. 
or cyclical portion of its disk can in no way be distinguished from that of OrlitoUtes ; 
and that the only difference between these two t}*pes which has any semblance of 
validity, is the absence in OrlitoUtes of those successive encasings of the central nucleus, 
the presence of which seems to be a constant featm’e in OrMculina. It is to be observed, 
however, that these successive encasings are due enthely to the extension of the later 
whorls of the spire over the earlier ; and they are no longer formed in OrhicuJina when 
the helical mode of growth gives place to the cyclical. Hence it seems not unfau’ to 
surmise that if the helical growth of an aberrant Orhifolites were to continue untd its 
spire had made several turns, instead of stopping before the completion of one, its 
nucleus would receive successive investments from successive whorls, just as in the 
typical Orhiculina ; and the only difference between these two t^’pes woidd thus vanish. 
On the other hand, if the helical growth of an Orhiculina were to give place to the 
cyclical at an unusually early period, the central nucleus would receive no investment, 
and would present the flatness by which that of Orhitolites is characterized when com- 
pared with that of the typical Orhiculina. 
241. I cannot but believe that such as may have followed me through the details of 
my previous descriptions, will be disposed to agree uith me in thinking it justifiable to 
assume that such a range of variation as to the period of the change in plan of growth, 
would be only analogous to that which both these tj'pes present in so many other par- 
ticulars; and hence that the idea of the derivation of Orhitolites and Orhiculina fr-om 
the same original is scarcely less probable than that of the derivation of the helical and 
cyclical types of Orhiculina., or of the simple and complex types of Orhitolites, from a 
common parentage ; — particularly since, as was formerly pointed out 90), both t^'pes 
present analogous modifications in geological time. 
242. Let us now apply the same mode of inquiry to Alveolina. It has been shown 
93, 94) that this organism is closely allied in every other respect than its geome- 
trical plan of growth to the types we have just been considering; the structure of the 
shell and its relations to the contained sarcode-body, and the relations of the segments 
of that body to each other and to the external world, being essentially the same in them 
all^. Now, however improbable it might seem at first sight, that an Orhitolites which 
extends itself as a flat or biconcave disk by successive concentric growths, and an 
Alveolina acquiring a fusiform shape by successive turns round a progressively elongating 
axis, should have had a common original, yet when the intermediate links are duly 
studied, a continuous gradation is found to be established. For, as has just been 
shown, a longer continuance of the helical mode of growth in which Orhitolites often 
commences, would really produce an Orhiculina with its centre so invested by successive 
* I may take this opportunity of stating that the description which I based on the examination of 
sections of the shell has been fully confirmed by the internal casts obtained by Messrs. Paekek and 
Eupeet Joxes from specimens whose chamhors had been filled by an infiltration of silicate of ii'ou 178) ; 
which casts most accurately represent tlie form of the sarcode-body and of its individual segments with 
their connecting stolons. 
