In the three preceding papers of this series descriptions have been given of the perignatliie girdles of familiar representatives of the three families into which the Holectypoida are at present divided. From the scanty evidence available, it would appear that in respect of this structure Plesiechinus is typical of the Pygasteridæ (even including Anorthopygus). The girtile of Holectypus seems essentially similar, thereby differing from that of the Discoidiinse; but as yet the perignathic structures of the Cretaceous Coenholectypus are unknown.