GRAPTOLITE.
To this class may probably belong the organic remains called " Gmptolite,s," which are exclusively and characteris tically Silurian fossils. A certain knowledge of their affinities would require examination of the soft parts ; and the family has long been extinct. Indications of the flexible consistency of the polypary, and M. Barrande's statement of the existence of a cylindrical canal in its axis, which he conjectures to have contained the common connecting tissue of the polypes, have weighed with the writer in placing the Graptolites provision ally in the present class of Polypi. The axis of the polypary is sometimes straight (fig. 3, 3), sometimes spiral (fig. 3, 6). The ordinary form, as given by the Graptolites priodon (fig. 3, 3), c. Protovirgularia dichotoma, WC.; Silurian, Dumfries. s. Oldhamia antiqua, Forbes ; Cambrian, Wicklow.
3. Graptolites priodon, Brun.; Silurian, Britain.
4. Didymograpsus Murchisoni, Beck ; L. Silurian, Wales. S. Diplograpsus folium, His.; L. Silurian, Britain.
6. Rastrites peregrinus, Barr ; Silurian, Bohemia.
7. Ccenites juniperinus, Eichw.; U. Silurian, Dudley. S. Ptilodictya lanceolate, Lonsd.; U. Silurian, Tortworth.
9. Archimedipora Archimedes, Leaner. ; Carboniferous, Kentucky. o. Ptilopora plume, M'C. ; Carboniferous, Ireland.
Fenestrella membranacea, Ph.; Carboniferous, Britain.
is serrated on one side only, and is found abundantly in the Cambrian or older Silurian beds of Scotland and Wales ; it occurs also in the Ludlow rocks. The double Graptolites (DiplograpsIts, fig. 3, 5, and Didymograpsus, fig. 3, 4) are Cam brian forms. Rastrites (fig. 3, 6) had the polypes only in one side, and they are less crowded: it characterises Barrande's divi sion E of the Lower Silurian beds of Bohemia, and has not yet been found in Britain. The Graptolites occur in argillaceous strata, especially in the mud-stones of Wales and Cumberland, and in the alum-slates of Sweden. These beds remind one of the mud bottoms in which the Virgularia and other long and slender graptolite forms of " Pennatulide' flourish in forest like crowds. The primeval Graptolite may have presented a more generalized polype structure than is now met with in the specially differentiated Sertularians and sea-pens.