354 Dr. H. A. Nicliolson on the Distribution in Time 



terized by the possession, though not exclusive, of the species 

 Didymograpsus geminus^ D.patulus {=D. Jiirundo^ Salt.), and 

 Dendrograpsus HalUanus^ of which the two former occur also 

 in the Alum Slates of Aher, in Sweden. 



The Upper Llandeilo rocks of Britain contain thirtj-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^ 

 CUmacograpsuSj 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. 

 angustifoliuSj D. palmeus^ D. tamariscus^ Graptolites 8edg- 

 wickii^ G. Sagittarius ^ G. lohiferus^ G. Nilssoni^ Bastrites pere- 

 grinus^ R. Linnoei^ and Dicranograpsus ramosus. 



The Caradoc rocks, as a rule, do not yield any Graptolites ; 

 but striking exceptions are found to this statement in 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 " etage E," 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 turricii- 

 latus^ G. BohemicuSj G. discretus^ Diplograpsus putillusj and 

 D. confertus. 



In the Caradoc beds in Ireland there are found, amongst 

 other species, Diplograpsus pristis^ D. mucronatus^ Didymo- 

 grapsus sextans^ Helicograpsus gracilis^ Graptolites Nilssonij 

 G. Sedgwickiij G. priodon^ Dendrograpsus flexuosus^ 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 



