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VII.—The Classification of the Phacopidæ

Published online by Cambridge University Press:  01 May 2009

F. R. Cowper Reed
F. R. Cowper Reed
Affiliation:
of the Sedgwick Museum, Cambridge.
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Extract

In North and South America and in South Africa there is a considerable variety of types of the Dalmaniles-branch in the Devonian period which have in many cases received distinctive subgeneric names; but species retaining the typical characters of the Silurian forms persist at any rate in North America. Many of these Devonian forms show incomplete second lateral furrows on the glabella, these furrows not reaching the axial furrows and causing a partial coalescence of the two middle lateral lobes. This tendency towards the fusion of the first and second lateral lobes of the glabella is a departure from the perfect segmentation found in typical Silurian members of Dalmanites, and has caused Clarke to group all such forms together into the subgenus or section Synphoria. This type of structure, as Van Ingen has recently shown, is not unknown amongst the Silurian species of Dalmanites in America, but it finds its most pronounced development in Devonian time and occurs in the groups Coronura, Corycephalus, Odontocephalus, and Probolium, all of which are put by Clarke in the section Synphoria. The marginal ornamentation and different processes on the pygidium and head-shield on which these four groups have been founded are scarcely of the same structural importance as the modifications of the glabellar segmentation. As in other families, the spinosity of these forms is the symbol of a last expiring effort before extinction.

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Type
Original Articles
Information
Geological Magazine , Volume 2 , Issue 5 , May 1905, pp. 224 - 228
Copyright
Copyright © Cambridge University Press 1905

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References

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page 224 note 2 Clarke, , “Low. Silur. Trilob. Minnesota”: Rep. Geol. Nat. Hist. Surv. Minn., vol. iii (1894), p. 732.Google Scholar

page 224 note 3 Ingen, Van: School of Mines Quarterly, Columbia University, vol. xxiii, Nov. 1901, No. 1, p. 67.Google Scholar

page 225 note 1 Barrois has recorded from the Silurian of the Pyrenees a species Dalmanites Gourdoni, possessing the short pygidium of 10–11 axial rings, like those in Div. II, but with the margin spinose (Ann. Soc. Géol. Nord, x, 1883, p. 151, pl. vi, fig. 1).

page 225 note 2 Kayser, : Zeitschr. deut. Geol. Gesell., Bd. xxxii (1880), p. 19.Google Scholar

page 225 note 3 Portlockia has no phylogenetic or morphological characters of sufficient value to separate it subgenerically from Acaste, as Salter recognised years ago.

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