RELATIONSHIP OF AMMOCGETES TO OSTRACODERMS 339 
known as the post-orbital plate, so invariably found. In Fig. 134, C, 
I have inserted (er.) the position of the membranous cranium of 
Ammoceetes, and it is immediately evident that the primordial 
cranium of the Osteostraci must occupy the exact position indicated 
by this median hard plate. For this very reason this median plate 
would be harder than the rest in order to afford a better protection 
to the brain underneath. This plate, because of its position, may 
well receive the same name as the similar plate in the trilobite 
and various paleostracans and be called the glabellum, 
EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD—FORMATION 
OF CRANIUM. 
We may thus conceive the position of the nose, lateral eyes, 
median eyes, and cranium in these old fishes. In addition, other 
indications of a segmentation in this head-region have been found. 
The most striking of all the specimens hitherto discovered are some 
of Thyestes verrucosus, discovered by Rohon, in which the dorsal 
shield has been removed, and so we are able to see what that dorsal 
shield covered. 
In Fig. 136, I reproduce his drawing of one of his specimens from 
the dorsal and lateral aspects. These drawings show that the frontal 
part of the shield covered a markedly segmented part of the animal ; 
five distinct segments are visible apart from the median most anterior 
region, This segmented region is entirely confined to the prosomatic 
region, te. to the region innervated by the trigeminal nerve. An 
indication of similar markings is given in Lankester’s figure of 
Eukeraspis pustuliferus (see Fig. 127, B), and, indeed, evidence of 
a segmentation under the antero-lateral border of the head-shield 
is recognized at the present time, not only in the Cephalaspide, but 
also in the Pteraspidz, as was pointed out to me by Smith Woodward 
in the specimens at the British Museum. Also, in Cyathaspis, Jaekel 
has drawn attention to markings of a similar segmental nature 
(Fig. 137). 
There seems, then, little doubt but that these primitive fishes 
possessed something in this region which was of a segmental character, 
and indicated at least five segments, probably more. 
Rohon entitles his discovery ‘the segmentation of the primordial 
cranium.’ It would, I think, be better to call it the segmentation of 
