Part 2: Researches on the Foraminifera

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                                agreement and of difference. The general aspect of the
horizontal section of such a typical specimen as is shown
in Fug.16, does not differ from that of a corresponding section
of <u>Orbitolites</u> in any other essential particular than the dis-
position of the central portion, in which <s>a spiral is formed
by</s> the successive additions to the first-formed chambers are
so made as to produce a spiral, instead of a concentric disk.
When we examine this central portion more closely (Fig.
18), we see that its plan of growth is exactly the same as that
which is presented by those aberrant forms of <u>Orbitolites</u>,
in which the central cell buds-out only on one side, instead
of all around. (See [?]54, and Plate IX, Fig.4). The spiral mode
of increase is usually carried-on much further in <u>Orbi
culina</u>, than it ever is in <u>Orbitolites</u>; several turns being
made before it gives place to the cyclical plan. But as this
is a mere question of degree <s>the</s> such a difference would scarcely
<s>serve to [ob?]</s> alone afford a valid distinction between these two types.
There is, however, <s>a</s> this very definite positive distinction, <s>in
this,</s> - that in <u>Orbiculina</u> each turn of the spire not only
surrounds the preceding, but completely invests it above
and beneath, each band of new chambers being continued
to the very centre, so that, whilst the spiral mode of growth
continues, the thickness of the shell increases with each
turn; and after this <s>spiral mode of growth</s> has given place to the cyclical, the central nucleus, thus augment-
ed in thickness, projects above the plane of the disk. This
peculiarity in <u>Orbiculina</u>, which I have never found to
be wanting, and to which there is never the least approach
in <u>Orbitolites</u>, is best seen in vertical sections. Thus in

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



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