Chap. IX.] 
LOWEE SILUEIAN GEAPTOLITES. 
187 
number of characteristic fossils are common to the inferior and superior 
members of the system, and that the two are zoologically united through 
the intermediate ' Llandovery rocks.' In the course of the observations 
which follow, this generalization will be placed in a clearer point of view. 
In the earlier Chapters of this work, the fossils of the Longmynd rocks, 
the Lingula-zone, and the lower part of the Llandeilo strata have been much 
adverted to. In describing the succeeding formations, however, the typi- 
cal species alone have been cited ; and it is now desirable to give a more 
complete view of the general characters of the chief mass of the Lower 
Silurian fauna, or of the Llandeilo and Caradoc formations. 
Among the organic remains of the Lower Silurian rocks, Graptolites have 
been already often alluded to, being Zoophytes exclusively characteristic of the 
Silurian era (see p. 61) and which must have inhabited a sea with a muddy 
bottom. Of those forms which specially mark the Lower Silurian rocks, the 
Double Graptolite (Diplograpsus) and the Twin Graptolite (Didymograpsus) 
are the most common in Britain. Diplograpsus pristis, p. 51, Foss. 10. f. 15, ranges 
from the base of the Llandeilo flags to the Caradoc sandstone inclusive; D. 
folium, p. 61, Foss. 12. f. 6, is common in the anthracite-beds of Scotland (as 
well as in the Alum-slates, or Lingula-zone, of Sweden) ; and there are several 
other forms found in Scotland and Wales. 
There is a curious half-branched species, Diplograpsus ramosus, Hall, figured 
Foss. 12. f. 7, which seems to lead off to the Twin Graptolite, Didymograpsus, of 
which we have several species in England. Of these, D.Murchisoni,Beck,Foss. 12. 
f. 9, is one of our commonest Welsh species. The North-American forms, D. sex- 
tans of Hall (Foss. 12. f. 8), and D. MofFatensis, Carruthers, and the Continental 
D. Forchhammeri, Geinitz, are found in the South of Scotland; and there 
is yet another two-winged Graptolite (probably the same as a Canadian species, 
D. caduceus, Salter) in the slates of Wexford. 
A form of the North- American genus Phyllograpsus, Hall, which has a large 
axis and two or four very broad celluliferous plates, has been found in the Skid- 
daw Slates. The species that have a greatly produced and frequently branched 
stipe are represented by two species from the Llandeilo rocks — the one, Dendro- 
graptus linearis, Carr., from Scotland, the other, D. gracilis, Hall, from Ireland ; 
and a third species, D. involutus, Carr. MS., occurs in the Wenlock shales at 
Builth. The species with four, eight, or more branches proceeding from a single 
axis, which have been discovered by Sir W. E. Logan in Canada (Dichograpsus, 
Salter), are represented by three species from the Skiddaw Slates ; but no indica- 
tion of the singular disk that occurs in the Canadian species has been observed 
in any of them. 
Of the simpler forms, those with teeth on one side only of the branches, 
Eastrites and Graptolithus, the former is abundant in North Britain, where 
also the Graptolithus priodon, PI. XII. f. 1, occurs in plenty, though not quite 
so low in the series. This last fossil ranges from the Lower Silurian to the 
Upper Ludlow rock. 
Opinion has been divided among naturalists as to the true position of Grap- 
tolites in the animal kingdom. Of late they have been generally regarded either 
as Zoophytes of the Alcyonarian or the Sertularian orders, or as belonging to the 
Polyzoa (Bryozoa). If the organisms described by Mr. Nicholson (Geol. Mag. iii. 
p. 488) are really ovarian vesicles of Graptolites, this observation strengthens the 
