bones of the head, enamelled like the scales, are carved into jagged inequalities, somewhat resembling those on the skin of the shark, but more irregular. The sculpturings seem intended evidently for effect. To produce harmony of appearance between the scaly coat and the enamelled occipital plates of bone, the surfaces of the latter are relieved, where they border on the shoulders, into what seem scales, just as the dead walls of a building are sometimes, for the sake of uniformity, wrought into blind windows. The enamelled rays of the fins are finished, if I may so speak, after the same style. They lie thick upon one another as the fibres of a quill, and like these, too, they are imbricated on the sides, so that the edge of each seems jagged into a row of prickles. (See Plate VI., fig. 3.) The jaws of the Cheirolepis were armed with thickly-set sharp teeth, like those of its contemporaries, the _Osteolepis_ and _Diplopterus_.[W] [Footnote W: There have been five species of _Cheirolepis_ enumerated--_C. Cummingiæ_, _C. splendens_, _C. Traillii_, _C. unilateralis_ and _C. Uragus_. The _Cheirolepis splendens_ and _C. unilateralis_ Agassiz regards as doubtful.] [Illustration: _PLATE VI._] CHAPTER V. The Classifying Principle, and its Uses.--Three groups of Ichthyolites 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 Order.--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.