AGNATHA. 
57 
polygonal plates, of which some are again united into large 
bilaterally symmetrical or paired shields. Good examples 
of Drepanaspis (Fig. 52) are exhibited from the Lower 
r r 
Fig. 52. —Restoration of Drepanaspis gemuendenensis, upper (A) and \ lower 
(B) view, from the Lower Devonian of Bundenbach, Germany; one- 
quarter nat. size. a.v.l. anterior ventro-lateral; e.l. external labial; m. 
mental, behind mouth; m.d. median dorsal; m.v. median ventral; p.l. 
postero-lateral; p.v.l. posterior ventro-lateral; r. rostral; x. orbital 
plate with orbit. (After R. H. Traquair. Table-case B.) 
Devonian of Gmiinden in Ehenish Prussia; and there 
are numerous fragments of Psammosteus from the Upper 
Devonian of Scotland, N.W. Russia, and Spitzbergen. 
Sub-order 3. —Osteostraci. 
The Osteostraci (“ bony-shelled ”) differ from the Hetero- 
straci in the presence of bone-cells in some of the lower layers 
which unite the skin-tubercles into plates. They also differ 
in having the eyes close together on the top of the head. The 
Cephalaspidae are the best-known family, ranging Irom the 
Upper Silurian to the Middle or even Upper Devonian, but 
Table-case 
B. 
Table-cases 
B, C. 
