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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, Bead June 14, 1883. 



[PLATES 37,38.] 



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," t 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,"J 

 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. 

 t Phil. Trans., 1856, pp. 181-236. 

 J Ibid., Plate IV., figs. 5 and 6. 

 Ibid., Plate IX., figs. 1 to 4. 

 4 B 2 



