The Aru Archipelago consists of a group of some eighty low islands which rise from the shallow sea between western New Guinea and Australia. They have been interpreted by Wichmann (1887, pp. 120–1) as the south-eastern members of an arc, to which he assigned also the Sula, Obi, and Misool Islands, and part of the south-western coast of New Guinea. This arc he regarded as the easternmost of three arcs at the eastern end of the Banda Sea. This view has been adopted, amongst others, by Koto (1899, p. 97, pl. i). The Aru Islands consist, according to the account in Verbeek's “Molukken Verslag” (1908, pp. 9, 464–5, fig. 420), of an almost horizontal limestone plateau, which has been broken by uplift into more than eighty pieces.

In some recent papers dealing with Tithonian ammonites, the writer included zonal schemes of the higher Jurassic, up to what he considered the topmost horizon, namely the privasensis zone. The correlation of the Mediterranean ammonites with those of the “boreal province” or Pavlow's “Aquilonian” was also discussed. Since this author had recorded “Aquilonian” Craspedites and Garniericeras from the Speeton Clay and Spilsby Sandstone, the inquiry naturally led to a revision of the ammonites of the Neocomian of Yorkshire and Lincolnshire. A critical examination of the ammonite horizons at the Jurasso-Cretaceous border-line seemed specially invited because in our most recent textbooks Yorkshire strata obviously well up in the Cretaceous are still included in the Jurassic; and it was also deemed useful to link up the table of Tithonian ammonite zones, above referred to, with that of the Aptian, given by the writer in a paper on the “Ammonite Horizons of the Gault and Contiguous Formations”.