XIV. Researches on the foraminifera—supplemental memoir. On an abyssal type of the genus orbitolites;— a study in the theory of descent

• Published in: Philosophical Transactions of the Royal Society of London
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1883
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1098/rstl.1883.0014; https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1883.0014

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 sub-segments (formed by the division of the principal segments) at every new stage of gemmation; so that at last the alæ of the spire, extending themselves on either side round the nuclear mass, meet and complete the circlet, around which new zones are then successively budded forth, as in the forms that are cyclical from ...