188 ORIGINAL ARTICLES. 



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. 



Again, I have shown (2nd series) that a parallel variation is displayed 

 by the genus Orbiculina, whose ordinarily helical plan of growth has 

 caused M. D'Orbigny to range it among his Helicostegues, 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, is a fact of very high interest; 

 for when an Orbiculina has undergone this change, the outer or cyclical 

 portion of its disk can in no way be distinguished from that of Orbitoli- 

 tes ; and the only difference between these two types which has any 

 semblance of validity is the absence in Orbitolites 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 that they are no longer found in Orbiculina when the heli- 

 cal mode of growth gives place to the cyclical. Hence it seems not un- 

 fair to surmise that if the helical growth of an aberrant Orbitolites were 

 to continue until its spire had made several turns, instead of stopping 

 before the completion of one, its nucleus would receive successive in- 

 vestments 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 characterised as compared with that of 

 the typical Orbiculina. Hence the idea of the derivation of Orbitolites 

 and Orbiculina from the same original must be admitted to be scarcely 

 less probable than that of the derivation of the helical and the cyclical 

 forms of Orbiculina, or of the simple and complex types of Orbitolites, 

 from a common parentage. 



Let us now apply the same mode of inquiry to Alveolina. I have 

 shown (2nd series) thatthis organismis closely allied in every other respect 

 than its geometrical plan of growth to the types we have just been consi- 

 dering ; the structure of the shell and its relations to the contained 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. 



ISTow however improbable it may seem at first sight that an Orbi- 

 tolites, which extends itself as a flat or bi-concave disk by successive con- 

 centric growths, and an Alveolina acquiring a fusiform shape by 

 successive turns round a progressively elongating axis, should have 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 Orbitolites often commences, would really produce an Orbiculina, 

 with its centre so invested by successive whorls as to form a vertical 



