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DR. carpenter’s RESEARCHES ON THE FORAMINIFERA. 
same as that which is presented by those aberrant forms of Orhitolites, in which the 
central cell buds-out only on one side, instead of all around (see ^ 64, and Plate IX. 
fig. 4). The spiral mode of increase is usually carried-on much further in Orhiculina, 
than it ever is in Orhitolites-, several turns being made before it gives place to the 
cyclical plan. But as this is a mere question of degree, such a difference would 
scarcely alone afford a valid distinction between these two types. There is, however, 
this very definite positive distinction, — that in Orhiculina each turn of the spire not 
only surrounds the preceding, but eompletely invests it above and beneath, every 
band of new chambers being continued to the very centre; so that, whilst the spiral 
mode of growth continues, the thickness of the shell inereases with each turn ; and 
after this has given place to the cyelical, the central nucleus, thus augmented in 
thickness, projects above the plane of the disk. This peculiarity in Orhiculina, which 
I have never found to be wanting, and to which there is never the least approach in 
Orhitolites, is best seen in vertical sections. Thus in Plate XXIX. fig. 3 is shown 
that part of a vertical section of a large disk, which has passed through its nucleus ; 
the innermost and therefore first-formed portion of which is seen to be invested above 
and below by three layers, formed by three turns of the spiral. The same peculiarity 
is shown in Plate XXVIII. fig. 17, which represents a portion of a fossil disk near the 
nucleus. 
86. The transition from the spiral to the cyclical mode of growth is effected in 
Orhiculina, exactly as in the aberrant forms of Orhitolites just referred-to, by the 
opening-out (so to speak) of the mouth of the spire; the successive rows extending 
themselves more and more on either side, until they meet around the previously- 
formed portion ; after which each new row forms a complete zone or annulus. The 
commencement of this change is seen in Plate XXVIII. fig. 1 ; and its subsequent 
stages are shown in figs. 2-5, a comparison of which will show that the specimens 
which they represent are in different phases of this transitional state. 
87. But this transition by no means constantly occurs; for the original spiral plan 
of growth is not unfrequently maintained, apparently through the whole of life; so 
that specimens are often met-with, wliich are not inferior in size or in number of 
rows to the larger disks, but which retain the aduncal form. Such a series of speci- 
mens is shown in figs. 8-10 ; the first of which represents a very young Orhiculina, in 
that stage which is common to both types of growth ; whilst it is obvious from a com- 
parison of this with the two following, that increase has continued to take place 
upon the same plan, each row that is put-forth from its predecessor terminating 
abruptly like it at its free extremity, without any such disposition to extend itself 
as would carry it round the nucleus so as to form a complete annulus, A horizontal 
section of such a specimen as is represented in fig. 10, is shown in fig. 12. — It may, 
of course, be urged that such a difference ought to be accounted sufficient to sepa- 
rate the spiral and the discoida.l types of Orhiculina, as two distinct species; but the 
following reasons appear to me quite sufficient to negative such a mode of viewing 
