Part 2: Researches on the Foraminifera

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                                Fig.22 is shown that part of a vertical section of a large
disk, which has passed through its <s>central</s> 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.
86. The transition from the spiral to the cyclical mode of
growth is effected in <u>Orbiculina</u>, exactly as in the aber
rant forms of <u>Orbitolites</u> 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 com
mencement of this change is seen in Fig.18; its subse-
quent stages are shown on a smaller scale in Fig.16;
and a comparison of Figs 1 to 5 will show that the speci-
mens which they represent are in different phases of
this transitional stste.
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, which are not inferior in
size or in number of rows to the larger disks, but which
retain the [aduncal?] form. Such a series of specimens is
shown in Figs 8-11; the first of which represents a very
young <u>Orbiculina</u>, in that stage which is common to
both types of growth; whilst it is obvious from a compa
rison of this with the three following, that <u>their</u> increase
has <s>t</s> continued to take-place upon the same  plan, each
row that is put forth from the preceding abruptly termi

                            
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Manuscript details

Author
William Benjamin Carpenter
Reference
PT/53/8
Series
PT
Date
1856
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Part 2: Researches on the Foraminifera, 1856. From The Royal Society, PT/53/8

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