The shifting of the orifice of the olfactory passage, which led to the old mouth, from the ventral to the dorsal side, as seen in the transformation of the ventrally situated hypophysial tube of the young Ammocoetes, to the dorsally situated nasal tube of the full-grown Ammocoetes, affords one of the most important clues in the whole of this story of the origin of vertebrates; for, if Ammocoetes is the nearest living representative of the first-formed fishes, then we ought to expect to find that the dorsal head-shield of such fishes is differentiated from that of the contemporary Palæostraca by the presence of a median frontal opening anterior to the eyes. Conversely, if such median nasal orifice is found to be a marked characteristic of the group, in combination with lateral and median eyes, as in Ammocoetes, then we have strong reasons for interpreting these head-shields by reference to the head of Ammocoetes. The oldest known fishes belong to a large group of strange forms which inhabited the Silurian and Devonian seas, classed together by Smith Woodward under the name of Ostracodermi. These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ, to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis, Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong to the Upper Silurian, while the third is Devonian. THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI. Of the three orders above-named, the Heterostraci and Osteostraci are the oldest, and among them the Cephalaspidæ have afforded the most numerous and best worked-out specimens. At Rootziküll, in the island of Oesel, the form known as _Thyestes (Auchenaspis) verrucosus_ is especially plentiful, being found thickly present in among the masses of Eurypterid remains, which give the name to the deposit. Of late years this species has been especially worked at by Rohon, and many beautiful specimens have been figured by him, so that a considerable advance has been made in our knowledge since Pander, Eichwald, Huxley, Lankester, and Schmidt studied these most interesting primitive forms. All observers agree that the head-region of these fishes was covered by a dorsal and ventral head-shield, while the body-region was in most cases unknown, or, as in Eichwald's specimens, and in the specimens figured in Lankester and Smith Woodward's memoirs, was made up of segments which were not vertebral in character, but formed an aponeurotic skeleton, being the hardened aponeuroses between the body-muscles. This body-skeleton, which possesses its exact counterpart in Ammocoetes, will be considered more fully when I discuss the origin of the spinal region of the vertebrate.