GENUS ORBITOLITES: VARIATIONS IN MODE OF GROWTH. 219 



the stolons pullulate from the entire circumference of the nucleus, and the annular 

 zones of segments are complete from the first. The greater the limitation of the 

 power of gemmation to one side of the nucleus, and the larger the number of incom- 

 plete zones, the more will the early plan of growth approximate to the spiral type, 

 such as is represented in Plate IX. figs. 2, 4. It is obvious that the existence of 

 these intermediate gradations breaks-down that barrier between the extreme forms, 

 which Professor WILLIAMSON had proposed to erect ; and shows that in this, as in 

 many other particulars, differential characters, which at first sight appeared to be per- 

 fectly satisfactory, lose all their force when carefully traced through a sufficiently 

 extended series of specimens. 



56. It is desirable to note, as bearing on the relations between Orbitolites and 

 Orbiculina, that even in those forms of the first-named type, in which the spiral 

 mode of early growth is most characteristically displayed, it never seems to proceed 

 far beyond a single turn ; and further, that the later portion of this whorl merely 

 surrounds the earlier, and does not cover it ; so that unless (as sometimes happens, 

 Plate V. fig. 5) the nucleus should itself be thicker than the zones of cells which im- 

 mediately surround it, there is no central protuberance. In Orbiculina, on the other 

 hand, the early growth invariably takes place according to the spiral type ; this type 

 is always maintained, until several turns have been made ; and the later whorls not 

 only surround but cover-in the earlier, so as to give rise to the central knob or pro- 

 tuberance. Some general remarks, which I have to make on the combination of the 

 helical and cyclical types of growth, bearing upon certain fundamental questions of 

 classification, will be more appropriately introduced in a subsequent Memoir, after 

 the structure of Orbiculina shall have been compared with that of Orbitolites. 



57. It is not, however, in the early mode of development alone, that striking 

 diversities present themselves ; for numerous variations, some of them quite as 

 remarkable, are seen in the course of the evolution of the several parts which are 

 characteristic of the ' complex' type. Thus, in the first place, the intermediate stratum 

 is sometimes entirely deficient in the zones immediately surrounding the nucleus ; 

 so that the upper and lower annuli of sarcode are represented by only a single band, 

 as is indicated by the singleness of the aperture through which it passes. In the 

 specimen figured in Plate V. fig. 9, we see this to be the case only with the three zones 

 nearest the centre ; in that represented in Plate V. fig. 10, the canal is single in the 



Jive inner zones ; whilst in that represented in Plate V. fig. 7 5 the canal is single for 

 the first twenty-three zones. Whenever the annular canal is single, the upper and 

 lower superficial cells also become continuous, and form a series of columnar cells in 

 every respect similar to those of the simpler type (compare Plate V. fig. 5 with the 

 portion a b of Plate V. fig. 7). If, then, the growth of either of these disks had been 

 checked within the first zone in which its annular canal becomes double, it would 

 have been accounted as belonging to the simpler type ; and the wide variation which 

 here shows itself, in regard to the distance from the nucleus at which the more com- 



