366 PROFESSOR GEIKIE ON THE 
ciently recognised. As will be explained in the sequel of this memoir, there 
exist in that region representatives of the Caithness and Orkney beds covered 
probably unconformably by the true Upper Old Red Sandstone. The structure 
of the country, owing to paucity of sections, had not been satisfactorily deter- 
mined ; consequently some confusion could not fail to arise as to the true 
horizons of certain fossils. In particular, genera which elsewhere in this country 
are characteristic of Upper Old Red Sandstone were given as occurring in the 
same strata with characteristic forms of the true Caithness series. I shall again 
advert to this source of confusion. Prior to any comparison between the Old 
Red Sandstone of the north of Scotland and of Russia it is necessary to know 
precisely between what subdivisions of the system the comparison is to be made. 
In the second place, in the comparisons made by Murcuison between the 
fauna of the so-called Middle Old Red Sandstone of Scotland and that of the 
Middle Devonian rocks of the Continent, there was an obvious vagueness which, 
no doubt, led to its being “ slightingly spoken of,” as he himself remarked.* 
He nowhere, so far as I am aware, gave any list of the species of ichthyolites 
and of molluscs found together, but contented himself with citing the names of 
a few genera of fish which had been obtained in Russia associated with true 
Devonian shells. In the paleontological volume of the great work on “ Russia 
and the Ural Mountains,” AGassiz inserted a list of the various ichthyolites 
which up to that time (1845) had been obtained from the Old Red Sandstone 
of Europe. Eighteen species are there marked as occurring both in Scotland 
and in Russia. Of these, thirteen are forms belonging to the Upper Old Red 
Sandstone of Scotland. The remaining five (Osteolepis major, Diplopterus 
macrocephalus, Gilyptolepis leptopterus, Asterolepis Asmusii, and A. minor) 
occur among the shales and nodules of the Moray Firth and the flagstones of 
Caithness. It is impossible from Agassiz’ list or Murcuison’s descriptions to 
be certain whether or not these five species occur in the very same strata with 
the others. The Russian development of the Old Red Sandstone, however, 
though it covers so wide an area, consists evidently of very flat and little dis- 
turbed beds, and, if one may judge from the sections in “ Russia and the Ural 
Mountains,” does not attain a great thickness. It would appear to represent 
merely the upper part of the Old Red Sandstone of Britain. The occurrence 
in it of a few other types of fishes like those just cited, cannot be regarded as 
evidence sufficient to establish a “middle” Old Red Sandstone series in Britain. 
It may merely show that in the basin of the east of Europe certain species of 
fishes survived longer than they seem to have done in Lake Orcadie. 
Since the appearance of Murcutson’s paper in 1858, several interesting con- 
tributions have been made by other writers to the literature of the subject and 
* « Siluria,” 4th edit. p. 362. 

