rostral 

 N. \^ supraorbital conol 



dorsal plate 



rostral 



oral plates 



infraorbital canal 



orbital plate 

 orbital plate orogoniale 



dorsal sensory line 



ventral sensory line 

 laterol sensory line 



bronchial plate 



bronchial opening 



cornual plate (fin) 



dorsal spine 



pineal body 



orbital plate 

 pineal plate 

 rostral 



semicircular canals 



cornual plate (fin) 



orogoniale 

 'postorbital canal 1 bronchial plate bronchia! opening 



ventral sensory line 



Figure 5-32. Structure of Cepholothorax of Simopterospis primoevo, a heterostracan. A, lateral 

 view with main sensory canols shown; B, dorsal view; C, ventral view; D, dorsal view of cast of internal 

 surface of bony armor. (After Stensio, 1 958) 



were acquired by the fusion of small denticles into polygonal 

 plates and these into larger units. Not all agnaths tended to 

 develop a dermal skeleton and of those that did, the time 

 and sequence of fusion were variable. If a solid carapace 

 was to be formed, this was held off till full growth was 

 achieved. It the tendency was to develop supporting (or pro- 

 tective), shaped plates, these appeared increasingly earlier 

 in the life cycle to perform their functional service over a 



longer period. Acquisition of plated or solid armor was 

 sometimes followed by reduction and fragmentation into the 

 constituent polygonal units, or into the denticular subunits 

 of the latter. 



Evolution of the agnaths The variations observed in the 

 fossil agnaths suggest a very early radiation of types, two of 

 which, the petromyzontid and myxinid, have come down to 



138 • THE HEAD SKELETON OF FISHES 



