PLACOGANOIDEI 1 23 



it is the earliest known indication of a vertebrate animal. 

 Pteraspis Lloydii occurs in the lower " old red " of Britain. 



Genus Coccosteus (kokkos, berry ; osteon, bone). — If a 

 heterocercal fin were added in outline to the restoration of 

 the fish of this genus, given in fig. 45, a correct idea would be 

 given of the " old red" fossil, which, in the progress of its 

 reconstruction, has suggested so many strange notions of its 

 nature and affinities. 



The helmet and cuirass are firmly united, and there is 

 no trace of the jointed appendages, like pectoral fins, which 

 characterize Pterielitlujs. The unprotected part of the trunk 

 shows an ossification of the neural and hasinal spines, and of 

 their appendages, the rays of a " dorsal " and " anal " fin ; 

 and, by the analogy of Cephalaspis, the tail was most probably 

 terminated by an unecp:ial-lobed fin. The lower jaw is com- 

 posed of two rami, loosely connected at the symphysis ; so 

 that, when displaced, as they commonly are in crushed fossil 

 specimens, they gave the notion of the fish being provided 

 with laterally-working jaws, like those of the lobster. But, 

 in reality, the jaw worked vertically upon a fixed upper jaw ; 

 both jaws being provided with from ten to twelve teeth on 

 each side, anchylosed to the bone. 



An under-view of the cephalothoracic buckler of Coccosteus, 

 according to Dr. Pander's restoration, is given in fig. 46, showing 

 the sutures of most of the cephalic plates, and the external 

 surface of the plates of the plastron. 9, rostral plate ; 7, 

 premedian ; 5, median; 8, prelateral ; 6, lateral; 16 and 24, the 

 suborbital bone ; 15, prcventromedian ; behind the lozenge-shaped 

 ventromcdian, and on each side, are (22) the prerentrolateral 

 and (20) the post-^cntrolatcral. The same figures mark the 

 above plates in the side view (fig. 45), with the addition of 

 (12) the dorsomedian and (14) the post-dorsomedian. 



The blank space between the neural (») and haemal (//) 

 spines of the fossil endo-skeleton indicates the position of the 



