350 Dr. H. A. Nicholson on the Distribution in Time 



Llandeilo rocks of Wales, would seem to be referable to the 

 same species. Another form, viz. D.Jlexuosus^ Hall, has been 

 recognized by Mr. Baily in the Caradocs of Ireland. 



The genus Climacograpsus^ Hall {— Diplograpsus in part) 

 has a vertical range from the Lowest Llandeilo up to the Lower 

 Llandovery, having its maximum development in the Upper 

 Llandeilo and Caradoc groups. It will thus be seen that 

 Glimacograpsus is strictly coordinate in its range with the 

 limits of the Lower Silurian rocks, of which its members 

 may therefore be considered characteristic fossils. The most 

 typical and familiar species, G. teretiiisculus^ His., has the 

 same range as the genus. It commences in the upper beds of 

 the Skiddaw Slates, is of tolerably frequent occurrence in the 

 Lower Llandeilo rocks of Wales, attains its maximum of 

 abundance in the Upper Llandeilo and Caradoc groups, and 

 is known in the Lower Llandovery by a single specimen only, 

 which was discovered by Prof. Harkness. A second species, 

 viz. G. antennarius^ Hall, also occurs in the Skiddaw Slates ; 

 but it appears to have died out before the deposition of even 

 the upper beds of this formation, and it is not known to occur 

 in any of the higher groups. Of the remaining species, G. 

 hicornisj Hall, occurs in the Lower Llandeilo rocks, but is 

 much more frequently found in the Upper Llandeilos ; whilst 

 G. tuherculatus^ Nich. (MS.), is confined to the last-named 

 group. 



The closely allied genus Diplograpsus^ M^Coy, is, like the 

 last, exclusively confined to, and essentially characteristic of, 

 the Lower Silurian period. Its range, however, is not quite so 

 wide, inasmuch as it is not yet known to transcend the limits 

 of the Caradoc series, though it commences as early as the 

 Skiddaw Slates. Of the British species, D. pristiniformisj 

 Hall, and D. mucronatus^ Hall, are the oldest, the former 

 being confined to the Skiddaw group, whilst the latter passes 

 up into the Upper Llandeilo, and in America occurs also in 

 the Caradocs* (Utica Slate and Hudson-River group). D. 

 Whitjieldiij Hall, D. cometa^ Gein., D, nodosus^ Harkn., D. 

 acuminatus^ Nich., and D. Harlcnessiij Nich., appear to be 

 exclusively Upper-Llandeilo species in Britain, the first pass- 

 ing up into the Caradocs in America. D.pristis^ His., D. an- 

 gustifoliusj Hall, D.palmeus^ Barr. (including D. folium^ His.), 

 D. tamariscus^ Nich., and D. vesiculosus^ Nich., are all cha- 

 racteristic fossils both in the Upper Llandeilo rocks and in the 

 Coniston Flags (Caradoc). D. ^^/i^7Z^^5, Hall, originally de- 

 scribed from the Utica Slate, and D, confertus^ Nich., are the 



* D. mucronatus appears also to occur in the Caradocs of Ireland. 



