CEPHALASPID^. 



15 



/3. Oculis extra scutum positis (?)/ 

 Genus 4. Thyestes. — Scutum simplex cornubus lateralibus parvis instructum. 



Figs. 1 — 7. Diagrams of the Shields of the several Genera of the Heterostracous 



and OsTEOSTRACOUS Cephalaspid^. 



R. Rostrum, or Rostral region. C. Cornua. D. Central disc, or Discal region. 5. Posterior spine. O. Orbits. 



Fig. 1. Fig. 2. Fig. 3. 



SCAPHASPIS. 



Ctathaspis. 



Fig. 5. 



Fig. 6. 



Cephaf-aspis. 



AUCHENASPIS. 



DiDYMASPlS. 



' The evidence as to the deficiency of the orbits in Thyestes is not very satisfactory. Should the 

 Russian Cephalaspirfce prove to have had their orbits placed mesially, the central position of the eyes 

 would become a common character of the Osteostraci. Thyestes might then be retained for a genus, 

 alhed to Cephalaspis, in which the posterior spine and cornua are rudimentary. Sir Philip Egerton's 

 Cephulaspis Murckisoni might then be joined with the Russian species in this genus. 



