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, 

 wrio-a-lincr 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 palaeostracan 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 Cephalaspidae, Tremataspida?, 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. In no case is there any sign of vertebra? 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. rostrcda, but the rostrum was longer and the spine at the 

 extremity of the head-shield much longer and more conspicuous. 



