CHAPTER II. 
The Armoured Primeval Vertebrates,— Subclass Ostracophori. 
Pteraspis. 
A GROUP of remarkable armoured forms from the Palasozoic 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- 
, . r .,, PARTIAL RESTORATION OP Pteraspis. 
centlic Stll8e. lne eyes (From A. S. Woodward, Cat. Foss. Fish. Brit. Mas.) 
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; 
