Abstract
The extensive series of specimens of members of the Hyolithidæ which Mrs Elizabeth Gray has collected from the Ordovician and Silurian beds of the Girvan district, and submitted to me for investigation, forms the basis of the following memoir.Through the kindness of Dr John Horne and Professor J. W. Gregory, special facilities have been afforded me of examining the other Scotch examples in the Geological Survey Collection, Edinburgh, and the Museum of Glasgow University. My thanks are also due to Dr F. L. Kitchin for the opportunity of studying the specimens in the Jermyn Street Museum.

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