deep into the body, as a ship's masts run deep into her hulk; and we can see them standing up among the scales to their termination, in such bold relief, that, from a sort of pictorial illusion, they seem as if fixed to the creature's sides, and foreshortened, instead of rising in profile from its back or belly. (See Plate VIII., fig. 1.) The observer will of course remember, that, in the living animal, the view of the spine must have terminated with the line of the profile, just as the view of a vessel's mast terminates with the deck, though the mast itself penetrates to the interior keel. Now, it must be deemed equally obvious, that, had the vertebral column been of bone, not of cartilage, instead of exhibiting no trace, even the faintest, of having ever existed, it would have stood out in as high relief as the internal buts or stocks of the spines. And such are the general characteristics of a few of the ichthyolites of this lower formation of the Old Red Sandstone--a few of the more striking forms, sculptured, if I may so speak, on the middle compartment of the Caithness pyramid. It would be easy rendering the list more complete at even the present stage, when the field is still so new that almost every laborer in it can exhibit genera and species unknown to his brother laborers. The remains of a species of _Holoptychius_ have been discovered low in the formation, at Orkney, by Dr. Traill; similar remains have been found in it at Gamrie. In its upper beds the specimens seem so different from those in the lower, that, in extensive collections made from the inferior strata of one locality, Agassiz has been unable to identify a single specimen with the specimens of collections made from the superior strata of another, though the genera are the same. Meanwhile there are heads and hands at work on the subject; Geology has become a Briareus; and I have little doubt that, in five years hence, this third portion of the Old Red Sandstone will be found to contain as many distinct varieties of fossil fish as the whole geological scale was known to contain fifteen years ago.[AA] [Footnote Z: Agassiz reckons four species of _Diplacanthus_--_D. crassispinus_, _D. longispinus_, _D. striatulus_, and _D. striatus_.] [Footnote AA: This prediction has been already more than accomplished. At the death of Cuvier, in 1832, there were but ninety-two species of fossil fish known to the geologist; Agassiz now enumerates one hundred and five species that belong to the Old Red Sandstone alone; and if we include doubtful species, on which he has not authoritatively decided--some of which, however, were included in the list of Cuvier--one hundred and fifty-one.]