364 PROFESSOR GEIKIE ON THE 
bearing Upper Old Red Sandstone. Consequently they must form a middle 
group by themselves. 
I propose to enter in some detail into the examination of the evidence on 
which this reasoning is based. In the meantime, I may remark that the pale- 
ontological discrepancy between the Old Red Sandstones of the north of Scot- 
land and that of the rest of the country, seems to be capable of reasonable 
explanation by isolation and differences in the conditions of deposit, and that 
it is really not so complete as is commonly supposed. 
In the first place, several genera, and perhaps species, are common to the 
Old Red Sandstone on both sides of the Highlands. The acanthodean fishes 
are eminently characteristic of the flagstones of Caithness and of Forfarshire. 
The genera Acanthodes and Diplacanthus abound ; and though the species found 
in the southern area are pronounced to be different from their congeners in the 
northern, they admittedly stand in close relation to each other. Parexus in- 
curvas is found in both tracts. Even the crustacean genus Pterygotus, which 
has been regarded as so characteristic of Lower Old Red Sandstone and Upper 
Silurian strata, occurs on two widely separated horizons among the Caithness 
and Orkney flagstones. So far as any comparison is possible at present be- 
tween the fossil plants, there appears to be a close similarity in the northern 
and more southerly regions. The lycopodian genus Psilophyton is common 
to both, besides lepidodendroid stems, and probably other still undetermined 
forms. 
In the second place, there does not seem to be any valid reason why the 
ichthyic fauna of two adjacent but completely disconnected water-basins should 
not have differed considerably in Old Red Sandstone times, as they do at the 
present day. Even in the same river-system it is well known that the fishes of 
the higher portions of the basin are sometimes far from corresponding with 
those in the maritime parts of the area. Neighbouring drainage-basins, divided 
by a comparatively unimportant watershed, sometimes show a remarkable con- 
trast in their fishes. This has been well pointed out by Professor E. D. Cops, 
in a suggestive paper “On the Distribution of Fresh-water Fishes in the Alle- 
ghany Region of South-Western Virginia.”* The James and Roanoke rivers 
descend the eastern slope of the continent and discharge into the Atlantic. In 
their upper waters they have only four species of fish in common. In the upper 
waters of the rivers Holston and Kanawha, which flow south-westwards into 
the Mississippi basin, there are only two species alike. Between those eastern 
and western pairs of rivers runs the more marked water-parting of the Alle- 
ghany chain. Out of fifty-six species of fish obtained from the head waters 
of the four rivers, five were found by Mr Cope on both sides of the water- 
shed. There is likewise considerable disparity in the genera represented in the 
* “Journ, Acad. Nat. Sci. Philadelphia,” vi. 2d series (1860-69), p. 207. 

