THE OLD RED SANDSTONE. 57 
ing disparted families which we find united in the inter- 
mediate families that no longer exist. Without some such 
preparation, the inquirer would inevitably share the fate of 
the poetical dreamer of Dunkeld, by losing his way in a laby- 
rinth. In passing, therefore, with this object from the ex- 
tinct to the recent, I venture to solicit, for a few paragraphs, 
the attention of the reader. 
Fishes, the fourth great class in point of rank in the ani- 
mal kingdom, and, in extent of territory, decidedly the first, 
are divided, as they exist in the present creation, into two dis- 
tinct series — the osseous and the cartilaginous. The osseous 
embraces that vast assemblage which naturalists describe as 
*‘ fishes properly so called,” and whose skeletons, like those 
of mammalia, birds, and reptiles, are composed chiefly of a 
calcareous earth pervading an organic base. Hence the du- 
rability of their remains. In the cartilaginous series, on the 
contrary, the skeleton contains scarce any of this earth: it is 
a framework of indurated animal matter, elastic, semi-trans- 
parent, yielding easily to the knife, and, like all mere animal 
substances, inevitably subject to decay. I have seen the 
huge cartilaginous skeleton of a shark lost in a mass of pu- 
trefaction in less than a fortnight. I have found the minutest 
bones of the osseous ichthyolites of the Lias entire after the 
lapse of unnumbered centuries. 
The two series do not seem to precede or follow one 
another in any such natural sequence as that in which the 
great classes of the animal kingdom are arranged. The 
mammifer takes precedence of the bird, the bird of the rep- 
tile, the reptile of the fish ; there is progression in the scale 
—the arrangement of the classes is consecutive, not paral- 
lel. But in this great division there is no such progression 3 
the osseous fish takes no precedence of the cartilaginous fish, 
