VI.—Note on the Genera Tristychius and Ptychacanthus, Agassiz

Abstract

Some years ago, in showing that the spines supposed by Hancock and Atthey to be dorsal spines of Gyracanthus were merely young and unworn specimens of lateral spines of the same genus, I called attention to the general fact that young examples of Selachian spines could not be expected to represent the older ones in miniature, and vice versa.