[Illustration: 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.)] [Illustration: 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 Ammocoetes 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 {341}where Schmidt and Rohon located it in Thyestes, viz. the so-called occipital region. This discovery of Rohon's is, in my opinion, of immense importance, for it indicates that, in these early fishes, the prosomatic segmentation, associated with the trigeminal nerve, was much more well-marked than in any fishes living in the present day. Why should it be more well-marked? Turning to the palæostracan, it is very suggestive to compare the markings on their prosomatic carapace with these markings. Again and again we find indications of segmentation in these fossils similar to those seen in the ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan _Bunodes_ and the fish _Thyestes_, both life size. In the latter I have indicated Rohon's segments; in the former the markings usually seen.