III.—On the Distribution of the Graptolithidæ in Time and Space

Extract

It was indicated even by Murchison that the Graptolites constitute admirable characteristic fossils of the Silurian formation. Subsequent investigation has established that the group Graptolithidæ is essentially confined to the oldest fossiliferous formation. A single genus, the genus Dictyograptus, Hopk. (Dictyonema, Hall), occupies a remarkably exceptional position as regards its distribution in time. Formerly, indeed, this genus was separated from the proper or true Graptolites (Rhabdophora, Allman), and referred with some other genera (Dendrograptus, Hall, Ptilograptus, Hopk., Callogroptus, Hall) to the Campanularidæ but recently W. C. Brögger has very clearly shown that the genus in question differs very little from the true Graptolites, inasmuch as the most important parts of the latter, such as the sicula, and the hydrothecæ, have been detected in it. By this the Graptolithic nature of the genus in question is rendered very probable. Members of the genus Dictyograptus, Hopk., appear among the very oldest of known Graptolites; the genus maintains itself throughout the whole of the Silurian formation, while by its side new genera make their appearance, culminate and disappear. Even after the other Graptolites had long since disappeared from the ancient sea-fauna, this genus still lived on, for we find it occurring in the Devonian.