256 On the Geological Distribution of the Rhabdophora. 
Species, of the Graptolites ” *, deriving many of the more 
important of his data from the results of his own extended 
researches. In this paper the range of the Rhabdophora was 
defined as extending from the base of the Skiddaw (Arenig) 
series to the summit of the Ludlow formation. The restric¬ 
tion of DichograptuSj Tetragraptus , and Phyllograptus to 
strata of Lower Ordovician date (Skiddaw) was pointed out, 
and the local stratigraphic position of many genera most 
carefully given. The general conclusions drawn by the author 
from the data before him are of great historical value, as 
showing the rapid advance in our knowledge of the distri¬ 
bution of these fossils within the last few years. To one who 
is at all familiar with the facts recently made known regarding 
the special horizons marked by these forms, it is interesting to 
read that Climacograptus and Diplograptus are exclusively 
Ordovician genera, that Rastrites and Cgrtograptus are un¬ 
known above the Caradoc rocks, and that the genus Mono - 
graptus (Graptolites) ranges from the Skiddaw Slate to the 
summit of the Silurian. 
In his 1 Monograph of the British Graptolitidas,’ published 
in 1872, Professor Nicholson treats of the geological distribu¬ 
tion of the Rhabdophora in some detail. The graptolitiferous 
Lower Palaeozoics of Britain are regarded by him as belonging 
to three successive periods. To the first of the periods (the 
Skiddaw or Arenig period) are assigned the genera Trigono- 
graptus , Climacograptus , Diplograptus , and Didgmograptus 
and its allies. In the second period (that of the Llandeilo- 
Oaradoc formations) are placed the genera Dijdograptus , 
Climacograptus , Didgmograptus , Dicranograptus , Coenograp- 
tus , Pleurograptus , Cgrtograptus , Rastrites , Retiolites , and 
Monograptus. In the (Upper) Silurian the author only admits 
the presence of the genera Diplograptus and Climacograptus , 
but in Bohemia only, observing, u but for this as yet solitary 
exception, it might have been asserted that no diprionidian 
Graptolite, save the aberrant Retiolites , occurred in rocks 
younger than Murchison’s Lower Silurian ” f. 
The general tendencies of the foregoing conclusions, reg,d in 
the light of the favourite palasontogeological speculations of 
the time, are ably summed up by this author in his paper 
on the Migrations of the Graptolites, published during the 
same year J. 
In this memoir Dr. Nicholson accepts, without reserve, the 
generally received opinions with respect to the systematic 
* Ann. & Mag. Nat. Hist. 1868, ii. pp. 347-357. 
t Mon. Brit. Grapt. pp. 92-98. 
} Quart. Journ. Geol. Soc. London, 1872. 
