THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 3 1 



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 

 Rohon.) 



Fro., narial opening; i.e., lateral eyes; gl., 

 glabellum or plate over brain; Occ, oc- 

 cipital region. 



Fig. 15. — Ptcricthys. 



Cephalaspidian fish known by the name of Auchenasjris or Tlu/estes 

 verrucosa, the other a Palreostracan 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 



