G2 
SILUKIA. 
[Chap. III. 
group of these little serrated fossils has been called Graptolithina, and has 
been divided into several genera, as represented in the foregoing woodcut. 
One of these genera, Graptolithus, Foss. 12. f. 2, 3, has teeth or cells on 
one side only. A second, the Diplograpsus, M'Coy, f. 4*, 5, 6, is distin- 
guished by having a double series of lateral teeth. A third, Eastrites, 
Barrande, f. 1, has teeth placed like the first-mentioned, but not so crowded 
together. A fourth, Didymograpsus, M'Coy, has twin branches, f. 8, 9 : 
one of its species, D. Murchisonii, f. 9, is the most characteristic fossil in- 
the Llandeilo Flags of Wales. Some of these forms, as published in the 
' Silurian System,' are figured in PL I. and PL XII. The complex forms 
which have been more recently discovered constitute other genera, and 
will be described and figured in Appendix D. 
The Diplograpsi, or doubly serrated forms, are chiefly characteristic of 
the Lower Silurian rocks, D. pristis (D. foliaceus), PL I. f. 2, and D. 
folium, Foss. 12, f. 4* and 6, being ordinary forms. The one-sided species 
extend from the Lower to the Upper Silurian. 
"We now know that one of the species of this group, G. priodon, Bronn 
(G. Ludensis, Sil. Syst.), ranges from the Llandeilo formation to the Ludlow 
rocks inclusive, having been named ' Ludensis ' from its occurrence in the 
uppermost member of the Silurian series. In no Palaeozoic rock younger 
than the Silurian is, I repeat, the true Graptolite known. Hence, as types, 
they are most important to the practical geologist, who, in exploring many 
strata of this age in Cumberland, the southern counties of Scotland, Ire- 
land, Central Germany, <fcc, meets with scarcely any other fossils f . In 
Sweden, indeed, Graptolites and Fucoids so abound as to have given a 
highly bituminous character to the lower strata, which, being also largely 
impregnated with iron-pyrites, have afforded so much alum as to have 
procured for them the name of Alum-slates 
Whilst, however, the mere presence of a, Graptolite will at once decide 
that the enclosing rock is Silurian §, it is only by finding the genera of these 
animals which display a double set of serratures, as in several of the above 
figures, that, in the absence of signs of the order of superposition, the field- 
observer may presume he is examining the lower division of the system. 
t Dr. Beck of Copenhagen described the few Professor Hall inclines to believe them to be re- 
species of Graptolites in the ' Sil. Syst.' M. Bar- lated to Sertularians, as also do Professors M'Coy, 
rande has since published a most elaborate and Dana, and others. See Appendix D. 
valuable treatise on the Graptolites of his Silurian I See an important memoir by the late Dr. G. 
Basin in Bohemia. Dr. Geinitz, in 1852, systema- Forchhammer, on the formation of the Alum- 
tized all the then known forms of Graptolites, in slates of Sweden through the agency of Seaweeds, 
a work (illustrated by clear and beautiful draw- in the Eeport Brit. Assoc. 1844, p. 155 et seq. 
ings) entitled ' Die Versteinerungen der Grau- § It will be seen in the sequel, that these true 
wacken-Formation in Sachsen.' One of the most Silurian types occur in Norway and Sweden in 
important of the Memoirs on Graptolites is by strata of the same age as the Lingula-flags of 
Professor James Hall, in Decade II. of the Cana- Britain or the bottom beds of the series, 
dian Organic Remains, Geol. Surv. Canada, 1865. 
