THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 29 
who could possibly suggest such a connection. Much more likely 
is it that a fish-like form should have been developed out of a smooth, 
wriggling, worm-like animal, and it is therefore to the annelids that 
the upholders of the theory of the reversal of surfaces look for the 
ancestor of the vertebrate. 
We must endeavour to dismiss from our imagination such forms 
as the salmon and shark as representatives of the fish-tribe, and the 
lobster and spider of the arthropods, and try to picture the kind of 
animals living in the seas in the early Devonian and Upper Silurian 
times, and then we find, to our surprise, that instead of the contrast 
between fishes and arthropods being so striking as to make any 
comparison between the two seem an absurdity, the difficulty in the 
last century, and even now, is to decide in many cases whether a 
fossil is an arthropod or a fish. 
I have shown what kind of animal the palostracan was like. 
What information is there of the nature of the earliest vertebrate ? 
The most ancient fishes hitherto discovered have been classified 
by Lankester and Smith Woodward into the three orders, Hetero- 
straci, Osteostraci, and Antiarcha. Of these the Heterostraci contain 
the genera Pteraspis and Cyathaspis, and are the very earliest 
vertebrates yet discovered, being found in the Lower Silurian. The 
Osteostraci are divided into the Cephalaspide, Tremataspide, etc., 
and are found in the Upper Silurian and Devonian beds. The 
Antiarcha, comprising Pterichthys and Bothriolepis, belong to the 
Devonian and are not found in Silurian deposits. This, then, is the 
order of their appearance—Pteraspis, Cephalaspis, and Pterichthys. 
In none of these families is there any resemblance to an ordinary 
fish. Inno case is there any sign of vertebre 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 con- 
spicuous 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. 
