1899] ON THE ANATOMY OF FOSSIL PLANTS 371 



examination, shows many points of similarity, although its characters 

 are altogether more filicinean. Williamson and Scott describe in 

 detail two species {H. Grievii, Will, and H. tiliaeoides', Will.), 1 of the 

 former of which we now have the most complete knowledge, while the 

 detailed preservation of some parts of the latter is simply marvellous. 



This old type differs from Lyginodendron mainly in the primary 

 structure of the stele of the stem. There is no pith, but the whole 

 of the interior of the stele is occupied by the primary wood, consisting 

 of groups of tracheae interspersed with irregularly arranged bands 

 of parenchyma. The stem is therefore in general structure similar 

 to that of a typical monostelic fern such as Gleichenia. Unquestionably 

 Heterangium is much nearer the ferns than is Lyginodendron, but it is 

 most interesting to find, wdien we pass to the structure of the strands 

 of xylem which occupy the periphery of the primary wood, that here 

 again the primary bundles are mesarch, an arrangement which we have 

 already seen to be a cycadean one. 



In this case, again, as in Lyginodendron, most of the specimens 

 show a zone of secondary wood, which is, however, on the whole, less 

 developed than in the latter genus. The petioles and leaves (which 

 have been identified with Sphenopteris elcgans) also appear to have been 

 of the same fern-like type, both in form and structure, as those borne 

 on Lyginodendron. The root was also quite of the " Kalonylon " type. 



In H. tiliaeoides, Will., from the Halifax Coal Measures, which 

 received its specific name on account of a resemblance in some points 

 of its anatomy with the common lime-tree (Tilia), the tissues are most 

 beautifully preserved. Anatomical details, such as the cambium 

 elements and the sieve-tubes of the phloem, are here preserved, perhaps 

 better than in any other known fossil ; in some specimens even the 

 sieve-plates can be recognised with certainty. 



Both in Heterangium and in Lyginodendron, fern-sporangia have 

 been found in association with the remains, though never in actual 

 continuity with them. Nothing in the nature of a cone has ever been 

 found. The authors incline to the view that the material investigated 

 consists entirely of young plants, and in that way seek to account for 

 the absence of fructification ; considering that the whole question of 

 the nature of the fructification of these forms is best left open. 



All the preceding types have agreed in one important character, 

 that of monostely, there being but one vascular axis in the stem. 

 There is, however, an equally important series of polystelic forms now 

 known to belong to this intermediate group of Cycadofilices. These 

 are mainly known from foreign specimens, most of which exhibit a 

 very complicated stelar structure, and have been known for some years, 

 although Dr. Scott has quite recently described an interesting and very 

 much simpler form (Medullosa anglica) obtained from the Lower Coal 

 Measures of this country. 



1 ' : Further observations, etc." Part III., loc. cit. 



