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XIV. Researches on the Foraminifera—Supplemental Memoir. 
On an Abyssal type of the Genus Orbitolites ;— a Study in the Theory of Descent. 
By William B. Carpenter, C.B., M.D., LL.D., F.R.S. 
Received May 31,—Read June 14, 1883. 
[Plates 3 7 , 38.] 
Introduction. 
The subject of this communication is a type of the genus Orbitolites —first obtained, 
in the deep-sea dredgings of H.M. Surveying Ship “Porcupine,” off the north-west of 
Ireland, in 1869,* and subsequently brought up from various depths in other parts of 
the North Atlantic and also in the Mediterranean,—which presents many points of 
general scientific interest; the first of these being the completeness of the transition 
which it establishes between the Milioline and the Orbitoline plans of growth, and 
the full confirmation it thus affords of the validity of the principles on which my 
Classification of the Foraminifera is founded. 
In the Monograph of the genus Orbitolites (1855), which constituted the First Series 
of my “ Researches on the Foraminifera,”! I embodied the results of a careful and 
thorough investigation of the structure and relations of all the forms under which that 
type was then known to me : and I showed that while the most highly developed and 
most specialized of these forms exhibit the cyclical plan of growth almost from the 
very commencement,—a complete zone of sub-segments being formed by gemmation from 
the entire periphery of the “ circumambient segment ” of the central “ nuclear mass,”! 
and the whole disk being made up of a succession of similar concentric zones,—there 
are other forms in which the primary gemmation takes place from only one side of 
that mass, so as to impart to the early extension of the composite structure a more or 
less spiral direction,§ which only gives place to the cyclical after repeated gemmations. 
The transition from the one plan of growth to the other I showed to be made by 
the progressive widening-out of the spire, and the increase in the number of the 
* See Proc. Roy. Soc., vol. 18, p. 397. 
f Phil. Trans., 1856, pp. 181-236. 
t Ibid., Plate IV., figs. 5 and 6. 
§ Ibid., Plate IX., figs. 1 to 4. 
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