File:Report on the scientific results of the voyage of H.M.S. Challenger during the years 1873-76 - under the command of Captain George S. Nares, R.N., F.R.S. and Captain Frank Turle Thomson, R.N. (1887) (14596565477).jpg

Identifier: reportonscientif188721grea (find matches)
Title: Report on the scientific results of the voyage of H.M.S. Challenger during the years 1873-76 : under the command of Captain George S. Nares, R.N., F.R.S. and Captain Frank Turle Thomson, R.N.
Year: 1887 (1880s)
Authors: Great Britain. Challenger Office Challenger (Ship) Thomson, C. Wyville (Charles Wyville), Sir, 1830-1882 Murray, John, Sir, 1841-1914 Nares, George S. (George Strong), 1831-1915 Thomson, Frank Tourle
Subjects: Challenger Expedition, 1872-1876 Scientific expeditions
Publisher: Edinburgh : Neill
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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f fossil forms, an array of difficulties beset the task.Above all it is unfortunate that of the great majority of fossil forms only the dictyonalframework is known, so that there is no possibihty of induction as to the configuration ofthe soft parts, nor as to the form, number and disposition of the isolated skeletal elementswhich are so pre-eminently characteristic, and so extremely important, in spite of Zittelsopinion, in determination of relationship. Again, the blanks in the geological record arehere even more serious than in most of the other groups. From entire geologicalformations either no Hexactinellids are known, or only slight traces. Zittel indicates anobvious explanation in the following sentences:^—Our knowledge of the fossilHexactinellids is limited to isolated remains of a developmental series, widely separated(both in time and space) the links of which perhaps lie buried in the deposits now sunk > Pateontologie, i. pp. 199, 200. REPORT ON THE HEXACTINELLIDA. 495
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Fig. S.—Genealogical Tree of the Hexactiuellida. 496 THE VOYAGE OF H.M.S. CHALLENGEr.. beneath the sea-level, or in still unexplored regions. Under these circumstances it is notyet possible to construct a genealogical tree for the various genera, so we must dismiss toa pre-Silurian time, where the light of experience no longer guides us, all the hypotheseswhich would derive the Hexactinellida from the Tetractinellida or Monactincllida. An explanation of the sudden appearance of the fossil Lithistida3 and Hexactinellidamay be reasonably sought in the mode of life exliibited by their modern representatives.Both groups are especially inhabitants of the deep-sea, and only in the former deep-seadeposits can one hope to find their fossil remains in any abundance. If therefore we have to refer the origin of the Hexactinellida to a pre-Silurian periodof which no certain representatives are preserved, we cannot obviously expect frompalaeontology any conclusion as to the manner of their origi

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