GENUS ORBITOLITES: — VARIATIONS IN MODE OF GROVFTH. 
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 
Orhiculina, 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 Orhiculina, 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 Orhiculina shall have been compared with that of OrhitoUtes. 
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 Mree 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? 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 — h 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- 
