348 Dr. H. A. Nicholson on the Distribution in Time 
tion of Sir Roderick Murchison, that they are exclusively 
confined to this epoch, still holds good as far as all the typical 
forms of the order are concerned. The somewhat aberrant 
genus Dictyonema (which, along with some others, might fairly 
be placed in a distinct suborder) has been found by Hall in 
the Middle Old Red in America. This discovery would lead 
us to anticipate a similar revelation in Britain, whenever beds 
shall have been examined in this formation which present 
evidence of having offered the requisite conditions for the 
growth and preservation of these organisms. Further, the 
same genus Dictyonema occurs in the 'Tremadoc Slates, which 
are by some lookedupon as the top of the Upper Cambrian series. 
Nevertheless it remains certain that the Graptolites as a family 
are characteristically Silurian; and further researches are not 
likely to alter this statement in any essential point. Not only 
is this the case, but the Graptolites, as regards their abundance 
as individuals, and the number of generic and specific types, 
are far more characteristic of the Lower- than of the Upper- 
Silurian period. And, finally, the inferior portions of the 
Lower Silurian rocks can claim a decided predominance in the 
number of genera when compared with the superior members 
of the same. 
Contrary to what might have been expected, the various 
genera, and often the species also, of Graptolites are very 
constant in their range and distribution. ‘They afford, there- 
fore, very valuable and reliable data, whereby formations in 
different parts of the world may be correlated with one another 
or the exact position held by any group of beds in the strati- 
fied series may be more or less exactly ascertained. 
In Britain Graptolites are known to range from the Tremadoe 
Slates up to the Upper Ludlow rocks, inclusive, the Lower 
Llandeilo, Upper Llandeilo, and Caradoc groups being those 
in which there is the maximum development of genera and 
species, usually accompanied in the two latter cases by an 
extraordinary abundance of individuals. On the whole, the 
lower part of the Lower Llandeilo (Skiddaw Slates), as stated 
by Salter, must be looked upon as the “ metropolis” of the 
family, since it contains a larger number and a more varied 
series of generic types than is found in any other formation. 
The Upper Llandeilo and Caradoc groups, on the other hand, 
possess together a much greater number of species and of in- 
dividuals than is the case with the Lower Llandeilos, though 
this is, perhaps, largely due to the more favourable nature of 
their sediments, the same disproportion not being recognized 
in America. 
Looking merely to Britain, the Lower Silurian rocks are 
