572 DE. CABPENTEB'S EESEAECHES ON THE FOBAMINIFEEA. 



or cyclical portion of its disk can in no way be distinguished from that of OrMtolites ; 

 and that the only difference between these two types which has any semblance of 

 validity, is the absence in OrMtolites of those successive encasings of the central nucleus, 

 the presence of which seems to be a constant feature in Orbiculina. It is to be observed, 

 however, that these successive encasings are due entirely to the extension of the later 

 whorls of the spire over the earlier ; and they are no longer formed in Orbiculina when 

 the helical mode of growth gives place to the cyclical. Hence it seems not unfair to 

 surmise that if the helical growth of an aberrant OrMtolites were to continue until 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 Orbiculina ; and the only difference between these two types would thus vanish. 

 On the other hand, if the helical growth of an Orbiculina 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 Orbitolites is characterized when com- 

 pared with that of the typical Orbiculina. 



241. I cannot but believe that such as may have followed me through the details of 

 my previous descriptions, will be disposed to agree with 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 types present in so many other par- 

 ticulars ; and hence that the idea of the derivation of Orbitolites and Orbiculina from 

 the same original is scarcely less probable than that of the derivation of the helical and 

 cyclical types of Orbiculina, or of the simple and complex types of Orbitolites, from a 

 common parentage ; particularly since, as was formerly pointed out (^[ 90), both types 

 present analogous modifications in geological time. 



242. Let us now apply the same mode of inquiry to Aheolina. 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 OrMtolites which 

 extends itself as a flat or biconcave disk by successive concentric growths, and an 

 Aheolina 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 OrMtolites often 

 commences, would really produce an Orbiculina 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. PAEKEB and 

 EUPEIIX JOSES from specimens whose chambers had been fdled by an infiltration of silicate of iron (^J 178) ; 

 which casts most accurately represent the form of the sarcode-body and of its individual segments with 

 their connecting stolons. 



