198 THE STORY OF FISH LIFE. 



and these differed fundamentally from those of 

 all other vertebrates. Traces of eyes, nostrils 

 and gill apertures would only be discoverable 

 after careful search. There is reason to believe 

 that there were numerous gill-slits, but that they 

 opened, not directly on to the surface, but into a 

 common chamber below the head shield, and 

 that the water escaped from thence by a pair of 

 openings at the hinder end of this shield. 



There are three well marked types of these 

 ancient creatures distinguished by the structure 

 and form of the great shield enveloping the head 

 and upper part of the back, and hence called the 

 dorsal shield. All three types are generally in- 

 cluded in one common group, or sub-class, known 

 as the Ostracodermi, or shell - skinned animals, 

 but this grouping together is rather for the pur- 

 poses of convenience than to suggest any close 

 relationship. 



Those who would study these remains for 

 themselves in museum collections would find 

 these three groups arranged under three heads : 

 the Heterostraci (anomalous shells), Osteostraci 

 (bony shells), and the Antiarcha. 



The Heterostraci represent the simplest and 

 possibly the oldest of these groups. The head 

 shield, which may be seen in our illustration 

 (fig. 16 A), is made up of no less than seven 

 pieces marked by numerous concentric lines. 

 Each of the separate plates are believed to have 

 been caused by fusion of minute shagreen 

 tubercles. A section through the shield shows 

 it to consist of three layers — an inner, called the 

 " nacreous " layer, on account of its resemblance 



