PTERASPIDJE. 
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 Coelolepidre and closely 
related to the above, are described thus:— , 
s^xomjoJiodus sandelensis , C. H. Pander, op. cit. (1856), p. 76, pi. vi. 
figs. 15-17.—Tipper Silurian ; Oesel. AX. 
Nostolepis striata , C. H. Pander, ibid >t 'p. 68, pi. vi. fig. 7.—Ibid. XX \Ji fc>. 
Ms/i '■** 
- ' ppt /vi 9b'P<- "r 
e T'*™> /i:/ < > fj.t y ^ ^==4?,; 
3 f ? if y vofA ^ 
/Adn ** ■- 
to 
Subclass III. 0STRACODERMI. yr. ^ ' 
Exoskeleton well developed, the head and anterior portion of the 
trunk being covered with plates; month destitute of hard parts. 
/"* 
Arches for support of an appendicular skeleton rudimentary or 
absent. Notochord ^persistent, ^ 
A' 1 *’ 5 
Order I. H EXE ROSTRA CI. 
Exoskeleton consisting of calcifications without bone- corpuscles ; 
each plate comprising three superposed layers—an inner “nacreous” 
layer of lamellae, a relatively thick middle zone /6i 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, 
* Try An ^ ** *76- 
