204 



CEPHALASPIDOMORPHI 



large fin-like lobe, covered with scales continuous with the shield at its 

 base, projects from each side of the body behind the cephalic shield in 

 Cephalaspis (Fig. 173 [505, 276, 326]). 



Cephalaspis, Ag., and Auckenaspis, Ag. (Thyestes, Eich.) ; Upper Silurian 

 and Devonian, Europe. Eukeraspis, Lank., Silurian, and Didymaspis, 

 Lank., Devonian, England. 



FIG. 177. 



Diagrammatic restored outline of Ateleaspis tessdlata, Traq., the tail being turned so as to 

 appear in profile. The orbits are shown near the middle of the head-shield, and in front of 

 them the antorbital fossa and median pore. (After Traquair.) 



Family TREMATASPIDAE. The almost smooth cephalic shield is con- 

 tinuous round the sides on to the ventral surface. Ventrally, behind the 

 mouth, is an area covered by closely fitting plates, and between these and 

 the edge of the shield are a series of apertures, presumably branchial. 

 The interorbital plate is free ; the orbits roofless. No lateral flaps are 

 known [371, 325]. 



Tremataspis, Schmidt ; Upper Silurian, Russia (Fig. 176). 



Order 3. ANASPIDA. 



An order founded fry Traquair [466] for the reception of two 

 genera of very aberrant structure from the Silurian rocks of Scot- 

 land, Lasanius and Birkenia, to which has been added a less well- 



0. 



FIG. 178. 



Birkenia degans, Traq. ; restored. (After Traquair.) c./, caudal fin ; d.f, dorsal fin ; 

 /.o, lateral openings, possibly branchial ; o, orbit ; v.s, large ventral scales. 



