A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocœtes.

So close is this similarity, from the comparative point of view, between the dorsal head-shield of the Osteostraci and the dorsal cephalic region of Ammocœtes that it justifies us in taking Ammocœtes as the nearest living representative of such types; it is justifiable, therefore, to interpret by means of Ammocœtes the position of other organs in these forms. First and foremost is the hard plate known as the post-orbital plate, so invariably found. In Fig. [134], C, I have inserted (cr.) the position of the membranous cranium of Ammocœtes, 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 palæostracans 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, i.e. 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 Cephalaspidæ, but also in the Pteraspidæ, 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 the anterior region of the head, for that is in reality what his figures show, not the segmentation of the primordial cranium, which, to judge from Ammocœtes, was confined to the region of the glabellum.

What is the interpretation of this appearance?

Fig. 136.—Lateral and Dorsal Views of the Frontal and Occipital Regions of the Head-Shield of Thyestes, after Removal of the Outer Surface. (From Rohon.) Fig. 137.—Under Surface of Head-Shield of Cyathaspis. (From Jaekel.) A., lateral eyes; Ep., median eyes.

Any segmentation in the head-region must be indicative of segments belonging to the trigeminal or prosomatic region, or of segments belonging to the vagus or mesosomatic region. Many palæontologists, looking upon segmentation as indicative of gills and gill-slits, have attempted to interpret such markings as branchial segments, regardless of their position. As the figures show, they extend in front of the eyes and reach round to the front middle line, a position which is simply impossible for gills, but points directly to a segmentation connected with the trigeminal nerve. Comparison with Ammocœtes makes it plain enough that the markings in question are prosomatic in position, and that the gill-region must be sought for in the place where Schmidt and Rohon located it in Thyestes, viz. the so-called occipital region.