The earliest vertebrates of which we possess a workable knowledge occur in the Silurian, the Ordovician remains being extremely fragmentary. Of the Silurian forms none seem to be more primitive than the Anaspida, and among anaspidans the least specialized type is generally considered to be Lasanius, which, therefore, is of extreme interest as apparently the most primitive known fossil vertebrate. Traquair has described two species of this remarkable genus from the Downtonian of Lanarkshire, in Scotland—Lasanius problematicus and L. armatus.3 Both are of small size, from two to fifteen centimeters in length, if reconstructions from fragments may be trusted for the larger figure. The common form, L. problematicus, is usually about five centimeters long.
In his earlier paper (1898) Traquair published an outline restoration of the latter species, this being the reconstruction usually met in text-books. In 1905 he altered this slightly, making the animal abruptly larger back . . .