CHAPTER V. 
The Classifying Principle, and its Uses. —Three groups of Ichthyo- 
lites among the Organisms of the Lower Old Red Sandstone. — 
Peculiarities of the Third Group. —Its Varieties. — Description 
of the Cheiracanthus. — Of two unnamed Fossils of the same Or- 
der. — Microscopic Beauty of these Ancient Fish. — Various Styles 
of Ornament which obtain among them.— The Molluscs of the 
Formation. — Remarkable chiefly for the Union of Modern with 
Ancient Forms which they exhibit. — Its Vegetables. — Importance 
and Interest of the Record which it furnishes. 
THERE rests in the neighborhood of Cromarty, on the up- 
per stratum of one of the richest ichthyolite beds I have yet 
seen, a huge water-rolled boulder of granitic gneiss, which 
must have been a traveller, in some of the later periods of 
geological change, from a mountain range in the interior 
highlands of Ross-shire, more than sixty miles away. It is 
an uncouth looking mass, several tons in weight, with a flat 
upper surface, like that of a table; and as a table, when en- 
gaged in collecting my specimens, I have often found occa- 
sion to employ it. I have covered it over, times without 
number, with fragments of fossil fish — with plates, and scales, 
and jaws, and fins, and, when the search proved successful, 
with entire ichthyolites. Why did I always arrange them, 
almost without thinking of the matter, into three groups? 
Why, even when the mind was otherwise employed, did the 
fragments of the Coccosteus and Pterichthys come to occupy 
one corner of the stone, and those of the various fish just 
described another corner, and the equally well-marked re- 
mains of a yet different division a third corner? The 
(79) 
