G. W. Lee — Trepostomata. 
185 
it stands, the list of the species recorded here is fairly representative and suggests 
interesting points touching their distribution in this country, and may permit of 
comparisons with the corresponding faunas of foreign countries. 
The most outstanding feature of the British Carboniferous Trepostomata is the 
remarkably restricted distribution of the species, vertically and horizontally. Since most 
of the materials examined here form part of general assemblages of fossils systematically 
collected by skilled hands, the restricted distribution of the species cannot be solely 
attributable to the uncertainties necessarily inherent to fossil-collecting, and it seems that 
various physical causes such as the temperature and degree of purity of the water 
probably played a leading part in the distribution of these organisms. That the 
apparently restricted occurrence is real and not due to faulty observation seems to be the 
more probable when we consider the more conspicuous species, such as Tabulipora howsei 
(ISTich.) : such a striking object could not possibly fail to attract the attention even of a 
casual observer, so that the absence of that species—in England—South of Northumber¬ 
land is probably due to adverse physical conditions in that area. 
As regards the vertical distribution, it is not easy to estimate the parts played by 
physical causes and evolution respectively. The vertical range is narrow, and the species 
do not recur, although conditions favourable to the development of the Trepostomata 
generally, prevailed from time to time throughout the Lower Carboniferous sequence. 
As already stated in the first part of this memoir, so little is known of the ontogenetic 
development of the Trepostomata that we are not yet in possession of satisfactory means 
for showing the possible existence of genetic relationships between species occurring in 
successive zones. 
The distinguishing characters to which a specific importance is given are so salient 
that it is difficult to believe that they could be due to a process of gradual evolution. In 
that respect some light may be obtained by a comparison with the North American fauna. 
In North America there seems to be no great differences between the Lower and Upper 
Carboniferous related types, perhaps less than between any two British forms separated 
by a lapse of time much shorter than that between Lower and Upper Carboniferous. 
That is, there is lio evidence that in North America species originated by sudden 
mutational changes, and there is no apparent reason why the evolution of the forms 
should have proceeded differently in this country. The Trepostomata of Lower Carboni¬ 
ferous times cannot be said to be approaching extinction, so that the diversity of types 
does not seem to be attributable to racial degeneracy. Widespread species such as 
Stenopora redesdalensis and Tabulipora howsei are very stable, only a few evincing 
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