354 Dr. H. A. Nicholson on the Distribution in Time 
terized by the possession, though not exclusive, of the species 
Didymograpsus geminus, D. patulus (=D. hirundo, Salt.), and 
Dendrograpsus Hallianus, of which the two former occur also 
in the Alum Slates of Aher, in Sweden. 
The Upper Llandeilo rocks of Britain contain thirty-four 
species, belonging to ten genera. Of the whole number of 
species, sixteen, or nearly one-half, are common to the Coniston 
Flags, eleven are peculiar to this horizon, and thirteen are 
found in the Utica Slate and Hudson-River group in America. 
From this it will be seen that, taken as a whole, and as far as 
the Graptolites alone are concerned, it is impossible to make 
any separation between the Upper-Llandeilo and Caradoc pe- 
riods. The two periods, viewed together, are characterized by 
the great abundance of members of the genera Diplograpsus, 
Climacograpsus, Graptolites, Rastrites, and Dicranograpsus, 
of which the two latter do not occur in either younger or older 
formations. Amongst the species most highly characteristic 
of the two groups may be mentioned Diplograpsus pristis, D. 
angustifolius, D. palmeus, D. tamariscus, Graptolites Sedg- 
wickit, G. sagittarius, G. lobiferus, G. Nilssont, Rastrites pere- 
grinus, R. Linnet, and Dicranograpsus ramosus. 
The Caradoc rocks, as a rule, do not yield any Graptolites ; 
but striking exceptions are found to this statement im the 
Coniston Flags and in some of the Caradoc beds in Ireland. 
The Coniston Flags contain twenty-four species, belonging to 
six genera. Of these, six species occur in the Utica Slate and 
Hudson-River group of America, a decided relationship be- 
tween the two formations being thus established. A still more 
decided connexion is found to exist between the Graptolites of 
the Coniston Flags and those of Barrande’s “ étage H,” twelve 
of the species which occur in the former (constituting one- 
half of their entire number) being found in the latter also. 
Amongst the Graptolites which are not found in Britain else- 
where than in the Coniston Flags are Graptolites turricu- 
latus, G. Bohemicus, G. discretus, Diplograpsus putillus, and 
D. confertus. 
In the Caradoc beds im Ireland there are found, amongst 
other species, Diplograpsus pristis, D. mucronatus, Didymo- 
grapsus sextans, Helicograpsus gracilis, Graptolites Nilsson, 
G. Sedqwickit, G. priodon, Dendrograpsus flecuosus, and 
Callograpsus elegans. Most of these are common to the Upper 
Llandeilo rocks and Coniston Flags; the first four occur in 
the Utica Slate and Hudson-River group of America; and 
the last two are characteristic species in the Quebec group of 
Canada. 
_In the Lower Llandovery rocks one Graptolite only has 
