Phylogeny.

I have no hesitation in repeating a statement made in an earlier paper. The hypothesis that the modifications in the insertion-plates of Polyplacophora are due to the influence of ecological conditions over vast periods of time, and that these characters give us the best guide to the species proper place in the Natural Taxis, is increasingly substantiated the more I study this group of Mollusca. I am therefore the more willing to place confidence in those divisions that are based on such features.

In my “Monograph on Australian Fossil Polyplacophora (Chitons)” Proc. Roy. Vict. vol. 37 (n.s.) pt. 2, pp. 170–205, pls. 18–22, figs 1–36, I suggest that living Chitons have been evolved along two (at the least) distinct, parallel lines, having come to this conclusion as a result of my investigations in Australian Palaeontology. Up till the publication of the said paper it has generally been accepted that the Palaeozoic forms disappeared somewhere about the Jurassic, or earlier and the type that occur in later Secondary and Tertiary rocks are quite distinct, being the direct progenitors of living forms.

Owing to recent discoveries in the Oligocene (Balcombian) rocks of Victoria, I suggest that my new genus Protochiton forms one of the most important missing links and consider this species the progenitor of the Phylum Acanthochitonidae, that family having been derived from Palaeozoic stock along this line and not through the family Lepidopleuridae at all.

The genus Lepidopleurus has heretofore been considered the most primitive of all living forms, but it seems certain that the genus Protochiton cannot be derived from any member of that genus, for while some of its characters seem less primitive, others suggest an affinity with the Palaeozoic group, which does not exist in the Lepidopleuridae; I submit a Phylogenetic Diagram which will better express my views in this relationship.