In none of these families is there any resemblance to an ordinary fish. In no case is there any sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is their appearance, and it is no wonder that there should have been a difficulty in deciding whether they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield, especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis. In 1904, Drevermann discovered a mass of _Pteraspis Dunensis_ embedded in a single stone, showing the same kind of head-shield as _P. rostrata_, but the rostrum was longer and the spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is, then, natural enough for the earlier observers to have looked upon such a fossil as related to an arthropod rather than a fish. {30}[Illustration: FIG. 11.--_Pteraspis dunensis_ (from DREVERMANN). Dorsal view of body and spine on the right side. Head-end, showing long rostrum on the left side.] [Illustration: FIG. 12.--_Bunodes lunula._ (From SCHMIDT.)] [Illustration: FIG. 13.--_Auchenaspis (Thyestes) verrucosus_, natural size. (From WOODWARD.)] {31}In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same geological horizon. They are both life size and possess a general similarity of appearance, yet the one is a Cephalaspidian fish known by the name of _Auchenaspis_ or _Thyestes verrucosa_, the other a Palæostracan called _Bunodes lunula_. [Illustration: FIG. 14.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis) verrucosus_. (From ROHON.) _Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate over brain; _Occ._, occipital region.] [Illustration: FIG. 15.--_Pterichthys._] In a later chapter I propose to discuss the peculiarities and the nature of the head-shields of these earliest fishes, in connection with the question of the affinities of the animals which bore them. At this point of my argument I want simply to draw attention to the undoubted fact of the striking similarity in appearance between the {32}earliest fishes and members of the Palæostraca, the dominant race of arthropods which swarmed in the sea at the time: a similarity which could never have been suspected by any amount of investigation among living forms, but is immediately revealed when the ages themselves are questioned. [Illustration: FIG. 16.--_Bothriolepis._ (After PATTEN.) _An._, position of anus.]