[Illustration: FIG. 104.—_a_, Head-shield of an Upper Silurian fish (_Cyathaspis_). _b_, Spine of a Silurian Shark (_Onchus tenui-striatus_, Agass.). _c_, _d_, Scales of _Thecodus_, enlarged.] The first fishes that we certainly know are the Ganoids and Sharks, which appear near the close of the Upper Silurian, in the English Ludlow for example (Fig. 104). The Ganoids found here all belong to an extinct group, characterised by the covering of the head and anterior part of the body with large bony plates. They are mostly small fishes, and probably fed at the bottom, and used their long or rounded bony snouts for grubbing in the mud for food. In this respect they present a singular resemblance to the Trilobites, so that we seem to have here animals of an entirely new type, the Vertebrate, and with bony instead of shelly coverings, taking up the _rôle_ and, to some extent, the external form of a group about to pass away. Yet I presume that no derivationist would be hardy enough to affirm that the Trilobites could have been the ancestors of these fishes. Nor indeed is any ancestry even hypothetically known for them, for the doubtful Lampreys of the Cambrian Silurian are too remote and uncertain to be used in that way. The head-shield copied in outline in Fig. 104, and the restoration after Lankester in the frontispiece to this chapter, may serve to represent these curious primitive Ganoids, which are continued in the Devonian fishes represented in Figs. 105, 106. [Illustration: FIG. 105.—_Cephalaspis Dawsoni_ (Lankester). Lower Devonian of Gaspé.] Along with these, and not improbably their enemies, were certain Sharks (Fig. 104), known to us only by the spines which were attached to their fins as weapons of defence, and by detached bony tubercles which protected their skin. These remains are chiefly interesting as indications that two of the great leading divisions of the class of fishes originated together. In the Devonian age the Ganoids and Sharks, thus introduced in the Silurian, may be said to culminate. The former, more especially, are represented by a great variety of species, some of them nearly allied to their Silurian predecessors (Fig. 106), others of forms and structure not dissimilar to those of the few surviving representatives of the order, or altogether peculiar to the Devonian (Fig. 107). So numerous are these fishes, and of so many genera and species—and this not merely in one region, but in widely separated parts of the world—that the Devonian has not inaptly been called the reign of Ganoids. As an illustration at once of the very peculiar forms of some of these fishes and of their wide distribution, I figure here along with the British species a _Cephalaspis_ (Fig. 105) found in the Lower Devonian of Gaspé, in the same beds with some of the antique Devonian plants described in the last chapter. [Illustration: FIG. 106.—Devonian Placoganoid Fishes (_Pterichthys cornutus_, _Cephalaspis Lyelli_), from Scotland.]