A Long the branch which is marked earlier by Cyrtometopus and Cheirurus (sens. str.), the last stage of development corresponds with Salter's subgenus Crotalocephalus. This is almost entirely restricted to Devonian beds, but it is linked to Cheirurus (sens. str.) in time and zoological characters by Ch. Sternbergi (Boeck) and Ch. Quenstedti (Barr.), in which the glabellar side-furrows hardly meet in the middle of the glabella. The important subgeneric characters of Crotalocephalus are the continuous first and second side-furrows, the triangular basal lobes which nearly or quite meet in the centre of the glabella at their apices, and the nearly straight obliquely-directed third side-furrows of the glabella. The earlier forms of the subgenus have their triangular basal lobes still separated by a median portion of the glabella, as in Ch. Quenstedti, from Étage E. This species has a rather curiously specialized kind of pygidium, with only two pairs of long spinose pleuræ the third pair is very short and rudimentary, and of the fourth pair there is no trace. It is a noteworthy point that the pygidia of this subgenus are, as a rule, almost identical with those in the group of Cheirurus (sens. str.), with three equal pleuræ with free ends (vide antea), and do not show the enlargement of the first pair, as in the Russian Cheirurus (sens. str.).