II.—Studies in Edrioasteroidea. VI. Pyrgocystis n.g.

Abstract

Materials for the study of this very distinctive genus have been accumulating for many years, and at last afford a firm basis for certain definite conclusions. Some important matters remain obscure, and the comparison of the Swedish material has been hindered by the dangerous state of the North Sea, but further delay in publication does not seem advisable. Various reasons make it convenient to deal with the species in the order of their geological age, beginning with the oldest, which is the most complete and is taken as the type-species of the genus. The new species are— Pyrgocystis sardesoni, Lower Ordovician, Minnesota (genotype).Pyrgocystis grayae, Upper Ordovician, Girvan.Pyrgocystis ansticei, Middle Silurian, Shropshire.