This species, founded by Williamson on material from the Lower Coal Measures of Halifax, Yorkshire, while agreeing in the structure of the primary stele and in the general features of the cortex with the older _Heterangium Grievii_, is clearly distinguished by certain well-marked characters. Sclerous groups occur in the inner cortex as in _H. Grievii_ but they are present also in the pericycle. The peripheral leaf-traces in the stele show the mesarch structure rather more distinctly than in _H. Grievii_, and the secondary xylem, which forms a much broader cylinder than in the Scotch type, is divided by broad medullary rays into characteristic cuneate masses each of which rests at its base on the centrifugal tracheids of a leaf-trace strand of xylem (fig. 415, B). The most striking distinctive feature is afforded by the secondary phloem, which is often preserved in wonderful perfection; this is unusually thick and owing to the tangential expansion of the principal medullary rays the secondary phloem is divided into separate masses which decrease in breadth towards the external arcs of primary phloem. The triangular form of the phloem rays, composed of tangentially stretched parenchyma, suggested the specific name _tiliaeoides_ on account of their striking resemblance to the rays of _Tilia_. The leaf-traces are nearly always in pairs as they pass out through the cortex; they subsequently divide and appear as four vascular strands in the petiole. The portion of stem reproduced in fig. 415, _B_, 8 mm. broad, shows clearly the separation of the secondary xylem and phloem into wedge-shaped groups: in each group there are several narrow medullary rays. The extrastelar tissues are represented by a few fragments only. Several layers of crushed periderm occur in the pericyclic region but the more external tissues have been almost completely exfoliated[226]. [Illustration: Fig. 415. A, B, _Heterangium_. A, _H. Grievii_, _a_, leaf-traces. B, _H. tiliaeoides_. C, root of _Lyginopteris_. (A, B, Kidston Coll., 529, 294; C, Williamson Coll., 1631.)] Reference has already been made to _Heterangium Lomaxi_, the English type originally included by Williamson in _Heterangium Grievii_. The provisional species _Heterangium cylindricum_ Williamson and Scott[227] differs, as Scott says, in no important respect from _H. Lomaxi_ and should not be retained. A new species, _H. minimum_ Scott[228], has been founded on a very small stem from the Coal Measures of Dulesgate in which the leaf-traces leave the stele as single bundles as in the Scotch _H. Grievii_.