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III.—On the Organization and Economy of the Graptolithidæ
Published online by Cambridge University Press: 01 May 2009
Extract
In the first place, let us obtain a clear idea of a Graptolite, and for this purpose we will consider two members of the family Dichograptidæ, namely, Didymograptus vacillans, Tullb. (Fig. 2, p. 452), and Dichograptus octobrachiatus, Hall (Fig. 1, p. 449).
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References
page 450 note 1 Figures and Descriptions of Canadian Organic Remains, Dec. ii., Graptolites of the Quebec Group, Montreal, 1865.
page 450 note 2 Ueber Graptolitheu, etc., Breslau, 1851.
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page 453 note 1 The question of the angle of divergence appears to me to be a comparatively simple matter. Leaving out of consideration the two sides of the branches, the angle of divergence is that formed by the axial lines of the branches at their meeting-point in the sicula. Assuming the two branches to grow quite straight out from the sicula, there will be no angle of divergence (0°), and the celluliferous margins will be in contact;—then, as the branches diverge, sweeping round the imaginary circle of which the sicula is the centre (of course in the direction of the non-celluliferous margin, as otherwise they would have to cross each other), these axial lines will form gradually increasing angles, until the dorsal margins come close together, when we get 360° as in fig. 5. Roughly speaking, the angles shown in the figures would be, for fig. 2, about 90°; fig. 3, about 270°; fig. 4, 360°, and 330°; and for fig. 5, 360°.—W. S. D.
page 455 note 1 “Pterograptus, ett nytt Graptolitslägte,” Öfv. Kongl. Vet. Ak. Förh. 1881, No. 4.
page 457 note 1 Geological Survey of Canada: Graptolites of the Quebec Group, Montreal, 1865, and Introduction to the Study of the Graptolites, Albany, 1868.
page 458 note 1 Ann. Mag. Nat. Hist. Jan. 1882, pp. 54–57.
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