CEPHALASPIDOMORPHI 203 



the Cephalaspidae. Small scales like those of Thelodus are also 

 present in certain regions. 



In the Cephalaspidae the scales also extend on to the fins, 

 forming linear series closely resembling the lepidotrichia of Teleo- 

 stome fish ; and a ridge of strong scales is present along the back, 

 and along the sides of the trunk, which thus acquires a triangular 

 section (Fig. 174). The tail of the Tremataspidae is unknown, and 

 may have been naked. The expanded anterior region in the Atele- 

 aspidae is covered with a flexible armour of small polygonal plates ; 

 but in the Celphalaspid these fuse together over the head-region, so 

 as to form a large rigid dorsal shield with rounded anterior border, 

 and two posterior horns (Fig. 175). The compound origin of the 

 shield is betrayed by the persistence of polygonal areas, marked out 

 by vascular channels in the middle layer ; also by the presence of 

 an oval postorbital depression, and paired lateral spaces, in which 

 the polygonal plates remain separate (Fig. 173). These probably 

 indicate flexible areas over the brain and gill - chambers. The 

 cephalic shield of the Tremataspidae is still more thoroughly fused, 

 extending round to the ventral surface, so as to form an almost 

 complete box (Fig. 176). 



These dermal structures have probably been formed by the fusion 

 of small denticles and an underlying plate, as in the case of the 

 Pteraspids ; but the stages in the process have not yet been so 

 clearly demonstrated, nor has the presence of bone-cells in the 

 superficial layer been explained. 



The structure of the exoskeleton of the Cephalaspidomorphi is 

 now very thoroughly known, thanks to the labours of Lankester, 

 Rohon, A. S. Woodward, Patten, and Traquair. Yet their affini- 

 ties remain obscure. They differ markedly from the Pteraspidomorphi 

 in the position of the orbits, and in the presence of bone-cells in 

 the exoskeleton. The resemblance in shape between Thelodus and 

 Ateleaspis seems, however, to point to some relationship between the 

 two orders (Traquair [466]). 



Family Ateleaspidae. The head-region has a rounded margin 

 expanding into a rounded flap on each side. There are no shields, but 

 polygonal plates and scales, bearing sharp spinelets on their surface [466, 

 469]. 



Ateleaspis, Traquair; Upper Silurian, Scotland (Fig. 177). 



Family Cephalaspidae. The head -region is covered dorsally by 'a 

 large shield with inturned edge, and ventrally with polygonal scales. 

 The shield may bear a posterior median spine, and its posterior corners 

 may be produced backwards into immense cornua (Eukeraspis). In 

 Auclienaspis (Thyestes) the shield extends on to the trunk, several series 

 of trunk-scales having apparently been incorporated into its hinder region. 

 The interorbital plate is fixed. The cephalic shield is generally orna- 

 mented with tubercles, and may bear tooth-like spines along its edge. A 



