The history of the rocks shows that the group of fishes, Pteraspis and Cyathaspis, are older than the Cephalaspidæ--come, therefore, phylogenetically between the Palæostraca 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 Palæostraca, must have been ventral. The remarkable comparison which exists between the head-shields of Ammocoetes 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 Palæostraca 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 corresponded 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 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 corresponding 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 Pteraspidæ and Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and Bothriolepis), 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. {355}CHAPTER XI _THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE_