﻿PTERASPID^. 



159 



A scale from the Oesel limestone, differing only from the above in 

 the coarse crimping of two sides, is named Pachylepis costata, C. H. 

 Pander, op. cit. p. 67, pi. vi. fig. 9. 



Other dermal tubercles, apparently of the Ccelolepidae and closely 

 related to the above, are described thus: — 



Gomphodus sandelensis, C. H. Pander, op. cit. (1856), p. 76, pi. vi. 



figs. 15-17. — Upper Silurian ; Oesel. 

 Nostolepis striata, C. H. Pander, ibid. p. 68, pi. vi. fig. 7. — Ibid. 



Subclass III. OSTRACODERMI. 



Exoskeleton well developed, the head and anterior portion of the 

 trunk being covered with plates ; mouth destitute of hard parts. 

 Arches for support of an appendicular skeleton rudimentary or 

 absent. Notochord persistent. 



Order I. HETEROSTRACI. 



Exoskeleton consisting of calcifications without bone- corpuscles ; 

 each plate comprising three superposed layers — an inner "nacreous " 

 layer of lamella?, a relatively thick middle zone of polygonal 

 cancellae, and an outer hard layer of vaso-dentine. Dermal sense- 

 organs well-developed, arranged in canals traversing the middle 

 layer of the shield and opening by a double series of pores externally. 

 Dorsal shield of few pieces, firmly united in the adult; ventral 

 shield simple; [jaws never preserved] ; orbits wide apart and laterally 

 placed. Paired appendages absent. 



In this order is included a single family, that of the Pteraspidae. 



Family PTERASPIDAE. 



External layer of each dermal shield forming an ornament of very 

 fine, concentric, closely arranged ridges, parallel with the outer 

 margin. Rostral region relatively small. Scales of caudal region, 

 when present, numerous and rhomboidal. 



