As will be seen by reference to the figures on Plates li and lii, the Myonia of the Lower Marine of New South Wales differs markedly from the species so common in the Upper Marine. The former as a whole are characterized by smaller size and a tendency to develop thicker valves as compared with the Upper Marine types. This was due probably to the fact that the larger thin-valved types described in the first part lived in shallower and smoother waters, in some cases probably sub-estuarine. The same remarks apply equally to the members of the Chaenomya group occurring in the Upper and Lower Marines. The affinities of Myonia have been discussed by Mr. H. O. Fletcher.
It has been considered advisable to make a new genus, Pachymyonia, for Etheridge's Maeonia morrisii.