Part 2: Researches on the Foraminifera

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                                nating 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. 11, is
shown in Fig. 15. - It may, of course, be urged that such a
difference ought to be accounted sufficient to separate
the <u>spiral</u> and the <u>discoidal</u> types of Orbiculina, as two
distinct species; but the following reasons appear to me
quite sufficient to negative such a mode of viewing them.
- First, they are both precisely alike, so as to be indistin-
guishable, in their early condition. Second, they correspond
in every particular, so far as regards the<s>ir</s> structure of their minute parts. Thirds, the assumption of the cyclical
plan of growth does not take-place at any one fixed
epoch of development, but may occur at various periods.
Fourth, the persistence of the original plan of growth through
out life, cannot be fairly regarded as anything else than
an arrest of development, such as we shall presently see to
be of common occurrence in <u>Orbiculina</u>, as in <u>Orbitolites</u>, in regard to
other particulars.(S90)
88. Turning now from the general plan of growth
to the minute structure of the individual parts of <u>Orbiculina</u>, <s>we find</s>
we continue to find a very close conformity to the type of <u>Orbitolites</u>. The tex-
ture of the shell is precisely the same; and it exhibits no
other peculiarity than a minute punctuation of the super
ficial layers (Fig. 17<sup>d<sup>), which at first suggests the idea of
apertures, but which is found on careful examination
of transparent sections (Fig 21), to be due to a mere thinning of
the shell at certain points, so as to give an appearance of
                            
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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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