CHAPTER II. 



The Armoured Primeval Vertebrates, — Subclass Ostracophorl 



Pteraspis. 



A GROUP of remarkable armoured forms from the Palaeozoic rocks formerly 

 placed among the fishes, are now regarded as probably constituting a subclass of 

 the class typically represented by the lampreys, although it must be confessed 

 that we have at present no evidence of the links which, on this view, must be 

 supposed to have connected the two groups. Strange in form, and utterly unlike 

 any living animals, these primeval armoured vertebrates are characterised by the 

 great development of the external skeleton, the head and fore-part of the body 

 being protected by large bone-like plates. There are no hard structures to the 

 mouth, and there are, at most, but rudimental indications of arches for the support 

 of limbs, while the notochord is persistent. They may be divided into three chief 

 groups ; and one of the most curious features connected with the first of these is 

 the apparent structural identity of one layer of its shield with a layer found in 

 the investment of the living king-crabs. 



The simplest of these armoured forms is typified by the genus 

 Pteraspis of the Devonian rocks, a partial restoration of which is 

 given in the annexed figure. In these creatures the head and fore-part of the 

 body are protected both above and beneath by shields; while the tail, in some 

 cases at least, is scaled. The structure of the shield is curious, each plate consist- 

 ing of an outer and inner hard layer, between which is a thick stratum of 

 polygonal chambers, perforated by delicate tunnels of the sensory canal-system 

 of the skin ; all the 

 layers lacking the ele- 

 ments of true bone, and 

 the outer surface being 

 marked with fine con- 

 centric striae. The eyes 

 are lateral and widely 

 separated, and towards the hinder end of the back shield (which is provided with 

 a spine) there is an aperture on each side for the escape of water from a gill- 

 cavity. Although nothing is known as to the form and structure of the nose 

 and mouth, the nostrils must evidently have been placed near the mouth on the 

 under surface of the head. A pit between the eyes probably marks the site 

 of a rudimental median eye ; and the structure of the under surface of the shield 

 indicates the presence of separated gill-pouches, which were probably supported 

 by arches. From an examination of the whole structure of the remains, Mr. A. S. 

 Woodward is of opinion that the shield probably covered " a truly fish-like head ; 



PARTIAL RESTORATION OF Pteraspis. 



(From A. S. Woodward, Cat. Foss. Fish. Brit. Mus.) 



