The first account of the anatomy of _Halonia_ we owe to Dawes[312]; this was followed by a fuller description by Binney[313]. The history of our knowledge of this type of branch has been given by Carruthers[314], who expressed the opinion that Halonia is merely a fertile condition of _Lepidophloios_ and possibly of other lepidodendroid plants. He was also inclined to regard the Halonial tubercles as younger stages of the larger scars characteristic of the genus _Ulodendron_. Williamson’s contributions to our knowledge of _Halonia_ are of primary importance; he supplied further proof of the Lepidodendroid nature of these branches and advanced our knowledge of their anatomy. In an early paper[315] he expressed the view that the differences on which _Halonia_ and _Ulodendron_ are separated are such as result from a difference in age and are not of generic importance. In the last memoir, of which he was sole author, published by the Royal Society[316], Williamson brought forward further evidence in support of this well-founded opinion. That the fossils known as _Halonia_ are branches of a lepidodendroid plant is at least certain, and it is probable that the lateral branches which they bore were fertile, though satisfactory proof of this is lacking. We know also that Halonia branches are characterised by the Lepidophloios form of leaf-cushion; there is, however, no sufficient reason to assume that such branches were never attached to stems with the cushions of the Lepidodendron form. The further question, namely whether Williamson was correct in his contention as to the absence of any essential distinction between _Ulodendron_ and _Halonia_, does not admit of an unchallenged answer. In 1903 Weiss[317] described the anatomy of a specimen of a biseriate _Halonia_ branch of _Lepidophloios_. The form of the leaf-cushions is unfortunately not very well preserved, but Weiss figures other specimens with two rows of tubercles on which the leaf-cushions are sufficiently distinct to justify a comparison with those of _Lepidophloios_. He believes with Williamson that it is the presence of tubercles in place of scars which distinguishes _Halonia_ from _Ulodendron_, and that the arrangement of the tubercles or scars is a matter of little importance. He expresses the opinion justified by the evidence available that the absence or presence of tubercles is merely due to accidents of preservation or, one may add, to difference in age. Kidston[318] dissents from Weiss’s description of his specimen as a biseriate _Halonia_; he regards it as a Ulodendron branch of _Sigillaria discophora_ (König). Until specimens with more clearly preserved external features are forthcoming it is impossible to settle the point in dispute, but on the facts before us there would seem to be a _prima facie_ case in favour of Weiss’s contention.