The remarkable properties that synthetic resins possess make these substances of inestimable value in biological and geological laboratories and museums. Their applications in the biological sciences have been discussed by Hibbin (1937) and by Knight (1937). Bell (1939) and Shrock (1940) have indicated some of the geological methods employing polymerized methyl methacrylate; the material is remarkably transparent and this property tends to detract from the value of the plastic as a casting substance of fossils for, in such work, an opaque material is desirable.

It has been found in many places that the mineralogy of a series of sediments varies according to the time of its deposition; and basal beds may not contain the same minerals, particularly the heavy minerals, as the upper beds of a series. This investigation was undertaken to see if there was any such variation throughout the vertical thickness of part of the Permian of North-West Australia.

The relation of the valves to the pedicle is among the fundamental characters on the basis of which the phylum Brachiopoda has been divided into five orders (see Thomson, 1927). In a previous paper (Arber, 1939), I pointed out that, on this classification, the order Protremata cannot be regarded as a homogeneous group, since in the orthids the pedicle apparently emerged through an open delthyrium, while in the strophomenid Leptaena it emerged through an entirely separate foramen situated in the ventral valve. Schuchert in 1913 (p. 383) defined the whole superfamily Strophomenacea as being characterized by a pedicle which, when present, was small, and emerged at the apex of the valve. A pedicle-foramen is, however, by no means of universal occurrence, and its existence has also been often overlooked in systematic descriptions, so that, in spite of Schuchert's definition, some doubt has nevertheless remained as to whether the form of pedicle-opening found in Leptaena and a few closely allied genera is genuinely typical of the whole of the strophomenids, thus creating a developmental distinction between that group and the orthids. I have therefore examined as many as possible of the available specimens and descriptions of the strophomenids, with the results here recorded.