THE ASPIDOCEPHALI. 



359 



attached to the under side of mesocephalon, median to the cornua; flexible, with 

 terminal horizontal expansions. Trunk membranous, or covered with segment- 

 ally arranged plates, with twenty-five to thirty pairs of separately movable, appen- 

 dage-like, fringing processes. Lateral eyes, prominent, median. Caudal axis 

 straight, ending in slender filament. Ventral lobe of caudal fin, narrow and sub- 

 terminal. Upper Silurian to upper Devonian. 



Cephalaspis (Figs. 232, 233, 234); Eukeraspis (Fig. 235, D); Thyestis (Fig. 

 235, A); fringing plates (Fig. 233, C, D, E). 



Fig. 236. — Restoration of Tremataspis, seen from the neural surface, and showing the location of the principal sense 

 organs, canal organs, and appendages. X about i 1/2. 



Tremataspidae. — Dorsal and ventral shields of the mesocephalon and branch- 

 iocephalon united to form an oblong, lenticular buckler. Parietal eye plate free. 

 Small, 2-3 in. long. The best known form is Tremataspis Schmidti, whose 

 polished yellow shields are beautifully preserved in fine grained chalky rocks of 

 the upper Silurian in the island of Oesel, Baltic sea. 



Exoskeleton. — Divided into the usual three layers ; outer surface nearly smooth, 

 polished. Cancellae, somewhat irregular, small. Two sets of horizontal canals, 

 forming networks just below the outer layer. In one set, the canals are of varying 



