MIDDLE OLD RED SANDSTONE. 
443 
They contain Calamites and other plants which agree ge- 
nerically with Carboniferous forms. 
Middle Old Red Sandstone.-^In the northern part of Scot¬ 
land there occur a great series of bituminous schists and flag¬ 
stones, to the fossil flsh of which attention was first called by 
the late Hugh Miller. They were afterwards described by 
Agassiz, and the rocks containing them were examined by 
Sir R. Murchison and Professor Sedgwick, in Caithness, Crom¬ 
arty, Moray* Nairn, Gamrie in Banfl^*, and the Orkneys and 
Shetlands, in which great numbers of fossil fish have been 
found. These were at first supposed to be the oldest known 
vertebrate animals, as in Cromarty the beds in which they 
occur seem to form the base of the Old Red system resting 
almost immediately on the crystalline or metamorphic rocks. 
But in fact these fish-bearing beds, when they are traced from 
north to south, or to the central parts of Scotland, thin out, 
so that their relative age to the Lower Old Red Sandstone, 
presently to be mentioned, was not at first detected, the two 
formations not appearing in superposition in the same dis¬ 
trict. In Caithness, however, many hundred feet below the 
fish-zone of the middle division, remains of Pteraspis were 
found by Mr. Peach in 1861. This genus has never yet been 
found in either of the two higher divisions of the Old Red 
Sandstone, and confirms Sir R. Murchison’s previous suspicion 
that the rocks in which it occurs belong to the Lower “ Old 
Red,” or agree in age with the Arbroath paving-stone.* 
Fossil Fish of the Middle Old Fed Sandstone, —The De¬ 
vonian fish were referred by Agassiz to two of his great or¬ 
ders, namely, the Placoids and Ganoids. Of the first of these, 
which in the Recent period comprise the shark, the dog-fish, 
and the ray, no entire skeletons are preserved, but fin-spines, 
called Ichthyodorulites, and teeth occur. On such remains 
the genera Onchm^ Odontacanthiis^ and Ctenodus^ a supposed 
cestraciont, and some others, have been established. 
By far the greater number of the Old Red Sandstone fishes 
belong to a sub-order of Ganoids instituted by Huxley in 
1861, and for which he has proposed the name of Crossoptery- 
gidce,^ or the fringe-finned, in consideration of the peculiar 
manner in which the fin-rays of the paired fins are arranged 
so as to form a fringe round a central lobe, as in th^ Polyp- 
terus (see a. Fig. 499), a genus of which there are several spe¬ 
cies now inhabiting the Nile and other African rivers. The 
reader will at once recognize in Osteolepis (Fig. 500), one of 
the common fishes of the Old Red Sandstone, many points of 
* Siluria, 4th ed., p. 258. 
I Abridged from KpoocoTog^ crossotos, a fringe, and nrepv^^ pteryx, a fin. 
