III.—On Certain Genera of Living Fishes and their Fossil Affinities

On first thoughts, it may seems that the lowest group of vertebrates, of all the divisions comprised in the animal kingdom, might be most easily described, and its zoological limits defined; but, on examination, the fishes prove to be most curiously linked to the invertebrata below and the amphibian reptiles above. In fact, it is not easy to draw the lines positively between them, and to say where the true vertebrates begin, or where the piscine characters are merged in the reptilian. It is proposed to refer to some of the most aberrant forms of living fish and their fossil affinities; then, briefly passing in review the distribution of the various families in geological time, to see how far descent with modification is traceable in this class of vertebrates.