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 versa. — 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 

 Ammocoetes 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 

 Oyathaspis, are older than the Cephalaspidre — come, therefore, phylogenetically 

 between the Palreostraca 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 Palasostraca, must have been ventral. 



The remarkable comparison which exists between the head-shields of 

 Amniocoetes 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 

 marking's, found either in fossil Pakeostraca 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 Ammocoetes. 



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 trig-eminal 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 Pteraspidaj 

 and Cephalaspida?, as Avell as in the Asterolepida? (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. 



