354 THE ORIGIN OF VERTEBRATES 
question of their derivation from elasmobranch fishes ; for the main argument in 
favour of the latter derivation is the exceedingly strong one that bone succeeds 
cartilage—not vice versé—therefore, these forms, since their head-shield is bony, 
must have arisen from some other fishes with a cartilaginous skeleton, most 
probably of an elasmobranch nature. Seeing, however, that the structure of 
their shields resembles muco-cartilage much more closely than bone, and that 
Ammoceetes forms a head-shield of muco-cartilage closely resembling theirs, 
there is no longer any necessity to derive the jawless fishes from the gnatho- 
stomatous; but, on the contrary, we may look with certainty upon the Agnatha 
as the most primitive group from which the others have been derived. 
The history of the rocks shows that the group of fishes, Pteraspis and 
Cyathaspis, are older than the Cephalaspidaee—come, therefore, phylogenetically 
between the Paleostraca and the latter group. In this group the head- 
shields are of a very different character, without any sign of any structure 
comparable with that of bone, and although they possessed both lateral and 
median eyes, there is never in any case any trace of a dorsal nasal orifice. 
Their olfactory passage, like that of the Paleostraca, must have been ventral. 
The remarkable comparison which exists between the head-shields of 
Ammoccetes and Cephalaspis, enables us to locate the position of the brain and 
cranium of the latter with considerable accuracy, and so to compare the 
segmental markings found in many of these fossils with the corresponding 
markings, found either in fossil Paleostraca or on the head-carapaces of living 
scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial 
region was covered with a median plate, often especially hard, which corre- 
sponded to the glabellum of the trilobite; the growth of the cranium can be 
traced from its beginnings as the upturned lateral flanges of the plastron to the 
membranous cranium of Ammoceetes. 
From such a comparison it follows that the segments, found in the antero- 
lateral region of the head-shield, were not segments of the cranium, but of parts 
beyond the region of the cranium, and from their position must have been 
segments supplied by the trigeminal nerve, and not by the vagus group; 
segments, therefore, which did not indicate gills and gill-slits, but muscles, 
innervated by the trigeminal nerve; muscles which, as indicated by the corre- 
sponding markings on the carapace of Phrynus, Mygale, etc., were the tergo- 
coxal muscles of the prosomatic appendages. 
The discovery of the nature of these appendages in the Pteraspide 
and Cephalaspide, as well as in the Asterolepide (Pterichthys and Bothrio- 
lepis), is a problem of the future, though in the latter, not only have the 
well-known oar-like appendages been long since discovered, but Patten has 
recently found specimens of Bothriolepis which throw light on the anterior 
masticating gnathite-like appendages which these ancient forms possessed. 
