OSTRACODERMI OSTEOSTRACI 



529 



numerous primitively distinct polygonal plates (Fig. 319, A). 

 Between the orbits there is a separately calcified 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 



^7 r-S 





^•»S?' 





Fig. 319. — The dorsal shield of Cephalaspis lyelli (A), and an outline sketch of the 

 dorsal shield of Eukeraspis pustulifera (B). c, Postero-lateral cornu ; d, posterior 

 angle ; i.p, interorbital prominence ; o, orbit ; o.p, orbital prominence ; p.s, posterior 

 spine ; jj.v, postorbital 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 Cephalasins. The earliest remains 

 are found in the Ludlow Tilestones. The genus is also repre- 

 sented in the Ledbury Passage Beds, the Lower Old Eed 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 

 8|- 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 



' Traquair, Proc. Soy. Phys. Soc. Edinl. xii. 1894, p. 269. 

 VOL. Til 2 M 



