528 FISHES CHAP. 
hinder part invested by a rhombic squamation, the Osteostraci are 
distinguished by the presence of bone as a histological component of 
the dermal hard parts ; by the position of the orbits, which, instead 
of being marginal in position, are close together on the dorsal aspect 
of the carapace ; and by the possession of a median dorsal fin. 
Fam. 1. Ateleaspidae.'—The general shape of the body is 
much the same as in the. Coelolepidae, but the exoskeleton con- 
sists of numerous polygonal 
tuberculated plates in front of 
the postero-lateral lobes, and of 
‘sculptured rhombic scales be- 
hind. A pair of crescentic mark- 
ings, placed close together about 
the middle of the dorsal surface 
of the head, probably indicate 
the outer margins of orbital 
recesses (Fig.-317). The only 
species at present known (Afe/e-. 
aspis tessellata) occurs in the 
Downtonian beds. 
Fam. 2. Cephalaspidae.— 
In this family the dorsal shield 
is rounded in front, strongly 
~- a aa sketch of Ateleaspis arched above, with its postero- 
essellata. The crescentic markings 
indicate the position of the supposed lateral angles produced into 
erates Mien Ea) highly characteristic cornua (Fig. 
318). The shield consists of a single piece, but as the outer surface 
ea 
"RATSARTAREABS 
Y 
Fic. 318.—_Cephalaspis murchisoni. Upper Silurian and Lower Old Red Sandstone. 
op, Operculum (7). (From Smith Woodward. ) 
is ornamented by small tubercles arranged in polygonal areas, 
it is probable that it has been formed by the basal fusion of 
1 Traquair, Zrans. Roy. Soc. Edinb. xxxix. 1899, p. 834. 
° Lankester, Monogr. Palaeont. Soc. 1868 and 1870 ; Smith Woodward, Brit. Mus. 
Cat. Foss. Fishes, ii. 1891, p. 176. 
