RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 339 



known as the post-orbital plate, so invariably found. In Fig. 13-4, C, 

 I have inserted (cr.) the position of the membranous cranium of 

 Ammoccetes, 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 palseostracans 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 Thi/estes verrucosus, discovered by Eohon, 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 

 Hukeraspis pustuliferas (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 Cephalaspidee, but 

 also in the Pteraspidse, 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 



