nor the habits of the cephalopods of the palæozoic seas. But the day is probably not far distant when such doubts will be set at rest, and we shall know whether the nautili and orthoceratites swam in argosies over the surface of the ocean, or, keeping ever at the bottom, left the waves to roll far above them, unvaried save perchance by some floating sea-weed or drifted tree. [Footnote 40: And perhaps even that is doubtful, for it is not unlikely that after all, the palæozoic nautili may belong in reality to another genus. Twenty years hence will probably see no little change on our present identifications.] CHAPTER VII. Classification of the naturalist not always correspondent with the order of nature--Incongruous grouping of animals in the invertebrate division--Rudimentary skeleton of the cephalopods--Introduction of the vertebrate type into creation--Ichthyolites of the carboniferous rocks--Their state of keeping--Classification of fossil fishes--Placoids--Ichthyodorulites--Ganoids--Their structure exemplified in the megalichthys and holoptychius--Cranium of megalichthys--Its armature of scales--Microscopic structure of a scale--Skeleton of megalichthys--History of the discovery of the holoptychius--Confounded with megalichthys--External ornament of holoptychius--Its jaws and teeth--Microscopic structure of the teeth--Paucity of terrestrial fauna in coal measures--Insect remains--Relics of reptiles--Concluding summary of the characters of the carboniferous fauna--Results.