XX OSTRACODERMI—M—OSTEOSTRACI 529" 
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numerous primitively distinct polygonal plates (Fig. 319, A). 
Between the orbits there is a separately calcitied but fixed plate, 
which bears a hollow prominence, probably for the reception 
of a parietal organ. In some genera certain of the anterior 
dorsal and ventral scales of the trunk fuse into a continuous 
plate. Internally to the postero-lateral cornua the middle layer 
of the shield is prolonged backwards into a pair of singular flap- 
like lobes, which have been variously interpreted as corresponding 
to the lateral lobes of the Coelolepidae, to pectoral fins, or to 
Fie. 319.—The dorsal shield of Cephalaspis lyelli (A), and an outline sketch of the 
dorsal shield of Hukeraspis pustulifera (B). c, Postero-lateral cornu ; d, posterior 
angle ; 7.p, interorbital prominence ; 0, orbit ; 0.p, orbital prominence ; p.s, posterior 
spine ; p.v, pestorbital valley. (From Lankester. ) 
opercula. The scales of the trunk and tail are rhombic and 
imbricated ; on the sides of the body they are remarkably high 
and narrow. 
The best known genus is Cephalaspis. The earliest remains 
are found in the Ludlow Tilestones. The genus is also repre- 
sented in the Ledbury Passage Beds, the Lower Old Red Sandstone 
of Scotland, and the Upper and Lower Devonian of Canada. 
Most of the species are of small size, but C. magnifica, from the 
Caithness Flagstones,the largest of all the Cephalaspids, has a shield 
84 inches long, and 12 inches across the widest part. Auchen- 
aspis occurs in the Ludlow Tilestones and the Ledbury Passage 
Beds, and also in the Upper Silurian of the Isle of Oesel in the 
1 Traquair, Proc. Roy. Phys. Soc. Edinb. xii. 1894, p. 269. 
VOL. VII 2M 
