REL-ATIONSHIP OF AMMOCGTES TO OSTRACODERMS 349 
Paleostracan has already almost disappeared. At transformation 
the layers representing the stage arrived at by 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 Pteraspidz. 
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 branchie, The respiratory chamber, 
according to my view, had at first the double function of respiration 
aud 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 Paleo- 
straca. Many of these head-shields are remarkably well preserved, 
