350 = Dr. H. A. Nicholson on the Disabiain in Time 
Llandeilo rocks of Wales, would seem to be referable to the 
same species. Another form, viz. D. flexuosus, Hall, has been 
recognized by Mr. Baily in the Caradocs of Ireland. 
The genus Climacograpsus, Hall (= Dziplograpsus 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 
Climacograpsus 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, C. teretiusculus, 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. C. antennartus, 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, C. 
bicornis, Hall, occurs in the Lower Llandeilo rocks, but is 
much more frequently found in the Upper Llandeilos; whilst 
CO. tuberculatus, Nich. (MS.), is confined to the last-named 
roup. 
: 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. pristiniformis, 
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 Caradoes* (Utica Slate and Hudson-River group). D. 
Whitfieldit, Hall, D. cometa, Gein., D. nodosus, Harkn., D. 
acuminatus, Nich., and D. Harknessii, 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- 
gustifolius, 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. putillus, Hall, originally de- 
scribed from the Utica Slate, and D. confertus, Nich., are the 
* D. mucronatus appears also to occur in the Caradoes of Ireland. 
