340 THE ORIGIN OF VERTEBRATES 
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 Ammoccetes, was confined to the region of the glabellum. 
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 paleontologists, 
Fic. 136.—Larerat anp Dorsal Views Fic. 137.—Unp3ER SuRFACE oF HEAD- 
OF THE FRONTAL AND OcciPITaL REGIONS SHIELD OF CyaTHasPIs. (From 
OF THE HEeAD-SHIELD OF THYESTES,AFTER JAEKEL.) 
REMOVAL OF THE OUTER SURFACE, (From A., lateral eyes; Ep., median eyes. 
Rowon.) 
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 Ammocctes 
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 
