In Part I. of this series of memoirs (Phil. Trans. 1871, Plate 25, fig. 16, and Plate 27, fig. 39, p. 487) I described what appeared to be a transverse section of a Calamite, in which the woody wedges showed no traces of the longitudinal canal that occupies the innermost angle of each of these primary wedges in true Calamites. Not having at that time the materials which I have since accumulated, I was unable to say much about this exceptional specimen. I now know that it is not a Calamite, but a plant having a wholly different structure, and to which I propose assigning the provisional name of Astromyelon, from the peculiar stellate form which transverse sections of its pith exhibit. The plant had branching, unarticulated stems, a feature which at once distinguishes it from the Calamites, though transverse sections of the two plants exhibit such remarkable resemblances. It is one of the more common of the forms met with in the Oldham nodules. I have rarely seen specimens of it more than 0·25 in diameter. One example alone, represented in fig. 5, has had a diameter of nearly 0·75. Fig. 1 represents the more usual aspect of transverse sections of this plant enlarged 20 diameters. It consists of a central parenchymatous medulla, a , surrounded by an exogenous cylinder of vessels arranged in a very regular series of primary wedges, b , corresponding closely in all respects with those of Calamites, except in the absence of the long canals already referred to. The central cells of the medulla are much larger than those of the circumference, some of the former having a diameter of ·011, whilst the peripheral ones are not more than ·0041. In the longitudinal section, fig. 2, a , and fig. 3, a ', these medullary cells are seen to be somewhat elongated vertically, being often ·022 in length. The cells have usually rectangular partitions, and are arranged in vertical rows, as in many living ferns; one or two of the rows in immediate contact with the vascular zone (fig. 3, a ') have a similar arrangement, though they are much narrower in their transverse diameter. In the great majority of cases the pith is solid. In but a few instances have I found it otherwise. Two of the latter are represented in figs. 4 and 5. Fig. 5 further presents an example in which the medulla has had an unusually great diameter in proportion to that of the vascular zone. This exogenous zone is made up of a variable number of primary vascular wedges (figs. 1, b , 5, b ), each one of which is composed of numerous radiating vascular laminæ separated by medullary rays. The remarkable uniformity in their size and the regularity in the arrangement of these wedges gives to the transverse sections of the medulla the star-shaped outline already referred to. At their inner or medullary apex each of these wedges commences at a few vessels of somewhat larger size than those composing the rest of the vascular zone. These are observable in fig. 1, but they are much more conspicuous in some examples than in others. Fig. 7 represents a section, for which I am indebted to Mr. Butterworth, in which these vessels are extremely conspicuous. The medulla of this specimen is much disorganized by mineralization, but it has not been fistular. Its diameter is much less than is usually the case, contrasting strongly in this respect with fig. 5. In Mr. Binney’s monograph on the Calamites he figured, at page 20, what he believed to be part of a primary wedge of a Calamite, adding the remark, that in these Calamites “the wedge-shaped bundles of pseudo-vascular tissue originate from a small circular orifice or opening, sometimes simple, as in the specimen now under consideration, but in other instances apparently divided into several parts, as shown in the annexed woodcut (fig. 3).” I have no doubt that the latter examples were specimens of Astromyelon, which, like myself, Mr. Binney then mistook for Calamites.