CHAPTER II 



THE ARMORED PRIMEVAL VERTEBRATES SUBCLASS 

 OSTRACOPHORI 



A GROUP of remarkable armored forms from the Paleozoic 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 armored vertebrates are characterized by the great develop- 

 ment 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 struc- 

 tural identity of one layer of its shield with a layer found in the investment of the 

 living king crabs. 



The simplest of these armored 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 consisting 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 elements 

 of true bone, and the 

 outer surface being 

 marked with fine con- 

 centric s t r i ae . The 



PARTIAL RESTORATION OF Pteraspis. , . , 



eyes are lateral and 



(From A. S. Woodward, Cat. Foss. Fish. Rrit. J\fus.) 



widely separated, and 



toward 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 

 (2932) 



