RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 349 



Paheostracan lias already almost disappeared. At transformation 

 the layers representing the stage arrived at l»y the Heterostracan 

 and the Osteostracan disappear ; but the stage representing the 

 higher vertebrates, far from disappearing, by the formation of carti- 

 lage reaches a higher stage and prepares the way for the ultimate 

 stage of all — the formation of the bony cranium. 



So much for the evidence as to the nature of the structure of the 

 head-shield of the Pteraspidse. 



It suggests that these fishes were covered anteriorly with armoured 

 plates derived from the cutis layer of the skin, a layer which was 

 specially thickened and very vascular, apparently, to enable respi- 

 ration to be very largely, if not entirely, effected by the surface 

 of the body. It is difficult to understand how the sea-scorpions 

 breathed, and it is easy to see how the formation of ventral and 

 dorsal plates enclosing the mesosomatic appendages may at the outset 

 have hindered the action of the branchiae. The respiratory chamber, 

 according to my view, had at first the double function of respiration 

 and digestion. A new digestive apparatus was the pressing need at 

 the time ; it would, therefore, be of distinct advantage to remove, as 

 much as possible, the burden of respiration from this incipient 

 alimentary canal. 



What can be said as to the shape of these ancient forms of 

 fishes ? Certain parts of them are absolutely known, other parts are 

 guesswork. They are known to have possessed a dorsal shield, a 

 ventral shield formerly looked upon as belonging to a separate species, 

 called Scaphaspis, and a spine attached to the dorsal shield. The 

 rest of their configuration, as given in Smith Woodward's restoration 

 (Fig. 142) is guesswork ; the fish-like body with its scales, the hetero- 

 cercal tail, is based on the most insufficient evidence of something 

 of the nature of scales having being found near the head-plates. 



The dorsal shield is characterized by a pair of lateral eyes 

 situated on the edge of the shield, not as in Cephalaspis near the 

 middle line. In the middle line, where the rostrum meets the large 

 dorsal plate, median eyes were situated. But the slightest sign of 

 any median single nasal opening, such as is so characteristic of the 

 head-shield of the Osteostraci and of Ammoccetes has never been 

 discovered. The olfactory organ must have been situated on the 

 ventral side as in the larval stage of Ammoccetes, or in the Palaeo- 

 straca. Many of these head-shields are remarkably well preserved, 



