528 



FISHES 



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 [Atele- 

 aspis tessellata) occurs in the 

 Downtonian beds. 



Fam. 2. Cephalaspidae.' — 

 In this family the dorsal shield 

 is rounded in front, strongly 

 arched above, with its postero- 

 lateral angles produced into 

 highly characteristic cornua (Fig. 

 318). The shield consists of a single piece, but as the outer surface 



Fig. 317. — Outline sketch of Atdeaspis 

 tessellata. Tlie crescentic markings 

 indicate the position of the supposed 

 orbits. (From Traquair.) 



op. 



Fig. 318. — Cephcdaspis vi^irchisoni. 

 op, Operculum (? 



Upper Silurian and Lower Old Bed Sandstone. 

 (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 



^ Traquair, Trans. Roy. Soc. Edinb. .xxxix. 1899, p. 834. 



- Lankester, Monogr. Palaeont. Soc. 1868 and 1870 ; Smith Woodward, Brit. Mus. 

 Oat. Foss. Fishes, ii. 1891, p. 176. 



