THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 31 
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. 
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 
Fig. 14.—DorsaL HEaD-SHIELD oF Thy- 
estes (Auchenaspis) verrucosus, (From 
Ronon.) 
Fro., narial opening; 1e., lateral eyes ; gl., 
glabellum or plate over brain; Occ., oc- 
cipital region. 
Fig, 15.—Plericthys. 
Cephalaspidian fish known by the name of Auchenaspis or Thyestes 
verrucosa, the other a Paleeostracan called Bunodes lunula. 
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 
