Article: New evidence on Jamoytius kerwoodi White, an important ostracoderm from the Silurian of Lanarkshire, Scotland

Jamoytius kerwoodi White, based on two specimens from the Silurian of Lanarkshire, Scotland is reviewed in the light of many new specimens from the original locality. The associated fauna and probable habitat are discussed. Jamoytius is shown to be a cephalaspidomorph agnathan closely related to the anaspids and to the living cyclostomes. The presence is established of an annular cartilage, of sclerotic cartilages and of a cyclostome-like branchial basket. Lateral trunk scales of unusual structure and composition are present. Some further evidence is presented concerning the interpretation of continuous dorsal and lateral finfolds and the caudal fin is shown to be heterocercal (probably hypocercal). Jamoytius is thus a marine anaspid-like ostracoderm unspecialized enough to be on or near the line leading to the living cyclostomes, and could not, as was previously suggested, have represented the ancestral condition from which either the Cephalochordata or the Gnathostomata have arisen.