Table-cases 
A-E. 
Table-case 
A. 
Table-ease 
A. 
54 GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
an undergrowth of thin hard layers, bearing a superficial 
resemblance to ordinary shell. These are therefore named 
Ostracodermi (“ shell-skinned ”) or Ostracophori (“ shell- 
bearers ”). 
Sub-order 1. —Anaspida. 
A few small Ostracoderms of Upper Silurian age are 
laterally compresse d an d gracefully fusiform, with one small 
dorsal fin and a heterocercal tail (see p. 61). The hard 
skin-tubercles on their head are not fused into plates, but 
those on the trunk have coalesced into well-formed scales, 
arranged in rows which slope from behind forwards instead 
of the reverse or ordinary way. Birkenia (Fig. 47) and 
Lo.sanius occur in the Down ton ian formation of southern 
Fig. 47. —Restoration of Birkenia elegans, from the Downtonian of 
Lanarkshire ; about nat. size. (After R. H. Traquair. Table-case A.) 
Scotland, the former completely armoured, the latter showing 
only a few thick and deepened scales on the anterior part of 
the trunk besides a row of large recurved spines along 
the lower border. Euplianerops seems to have been a 
survivor of the Anaspida in the Upper Devonian of Canada. 
Sub-order 2. —Heterostraei. 
In this group of Ostracoderms the head and gill-chamber 
region is relatively large, broad and depressed, so that it is 
exposed from above or below in the fossils; while the tail is 
slender and, seen in side view, ending in a forked tail-fin. 
The eyes are wide apart on the sides of the head. The mouth 
must have been underneath the head, and the opening from 
the gill-chamber on each side is at the hinder angle of the 
expanded front part of the animal. When the hard skin- 
tubercles are fused into plates, the underlying layers never 
contain true bone-cells. 
The simplest of these Heterostraei (“ anomalous-shelled ”) 
