34o 



j 



THE ORIGIN OF VERTEBRATES 



the anterior region of the head, for that is in reality what his figures 

 show, nut the segmentation of the primordial cranium, which, to judge 

 from Ammocciites, was confined to the region of the glahellum. 



What is the interpretation of this appearance ? 



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 palaeontologists, 



Fig. 136. — Lateral and Dobsal 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., mcdiau eyes. 



looking upon segmentation as indicative of gills and gill-slits, have 

 attempted to interpret such markings as branchial segments, regard- 

 less 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 Ammocu'tes 

 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 



