Names underlined refer to hypothetical groups. _Mammalia_ _Aves_ | | _Reptilia_ | | | | +----------------+ | | | +--------------+ | _Proreptilia_ | _Amphibia_ _Pisces_ | | | | +----------+ | | | | | | _Dipnoi_ | _Stegocephali_ | | | | | +---------------+ | | +---------------+ | _Cyclostomata_ _Proselachii_ | | | _Tunicata_ | +--------+ | | | | *_Archicrania_* | | _Acrania_ | | | | *_Prospondylia_*------+ | | +----------+ | | | *_Prochordonia_* 11. Stage of _Cyclostomata_. This now small group of Lampreys and Hagfishes represents the lowest Craniota; and although much specialized as a side-branch of the main-stem from which the other Craniota have sprung, they give us an idea of what the direct ancestors of the latter must have been like:--still without visceral arches, without jaws and without paired limbs; with a persistent pronephros; the ear with one semicircular canal only; mouth suctorial; cranium very primitive; and the metamerism of the vertebral column indicated only by little blocks of cartilage in the perichordal sheath. Such creatures must have existed at least as early as the Lower Silurian epoch; but until 1890 fossil Cyclostomes were unknown. Their life in the mud, or as endoparasites of fishes, coupled with their soft structure, makes them very unfit for preservation. This gives all the greater importance to Traquair's discovery, in 1890, of many little creatures, called by him _Palæospondylus gunni_, in the Old Red Sandstone of Caithness, which seem to be very closely allied to Cyclostomata. 12. The _Elasmobranchi_ (sharks and skates), with their immediate forerunners, the Acanthodi of the Devonian and Carboniferous age, are the first typical fishes. That they existed as far back as the Silurian age is proved by many enamelled spines of the dermal armour, chiefly from the dorsal fins. This higher stage is characterized by the possession of typical jaws, by visceral or gill-bearing arches, and by two pairs of limbs. None of the Elasmobranchs, fossil or recent, stands in the direct ancestral line; but they are the lowest Gnathostomata, jaw-and-limb-possessing creatures, known.