In Marsupials the enamel of the teeth differs from that of most other Mammals in the presence of structures, continuous with tubules of the dentine, the point of junction often being somewhat enlarged. These structures pass outwards, usually pursuing a sinuous spiral course, and terminate a little before reaching the surface of the enamel, so that the outer zone of this tissue usually appears as a clear, structureless layer. On applying a strong acid the enamel is dissolved, and these structures alone remain attached to the dentine, from which, however, they readily part: they are resistant also to the action of alkalis, and from their behaviour to stains it is evident that they are organic. The structures in question were first described in 1847 by Sir John Tomes (l) as continuations of the tubes present in the dentine, and from that time onwards it has been usual to employ the word “tube” in describing them. C. Tomes (2), however, whilst retaining the word, is in doubt as to its correctness, and writes “whether, like the dentinal tubes, they consist of a resistant sheath containing a softer fibril, or whether they wholly consist of the resistant material, has not yet been shown.”