distinguish the structures characteristic of the subgenera of Sigillaria, or absolutely to separate these from those of certain peculiar conifers on the one hand and from those of the higher acrogens on the other. Young and succulent stems of Dadoxylon may have much resembled Sigillaria in their structure. Young stems of Sigillaria proper may have approached closely to those of Favularia;and since I have shown[1] that the branches of Favulariaresemble Clathraria in their scars, this last may have presented a still feebler type of internal organization. Further, there is, as I have already stated, reason to believe that some of the species referred by palaeobotanists to the Clathraria-division are really forms of Lepidophloios. These difficulties, in connexion with the defective state of preservation of specimens, may excuse many differences of opinion, though I think the facts already stated in this paper are sufficient to put all students of the subject on the right track in regard to at least one leading type of these plants, and to remove some of the more fruitful sources of error.
We may now proceed to inquire what light the structures of Sigillaria throw on its affinities. On this question, taken in its most general aspect, there have, I believe, in modern times been only two opinions, the views as to alliance with Euphorbioe and Cacti held by some older botanists having been given up. Some botanists, conspicuous among whom is Brongniart, hold that Sigillarioe were gymnospermous plants, allied to Cycadaceae. Others are disposed to regard them as acrogens, and as closely related to Lycopodiaceae.
In favour of the latter view may be urged the apparent association with Sigillaria of certain strobiles resembling those of Lepidophloios, the points of resemblance between the tissues of Favularia elegans and those of Lepidodendron, and the resemblance of certain Sigillaria, or supposed Sigillaria, of the Clathraria-type to Lepidophloios.
In favour of the former view, we may adduce the exogenous structure of the stem of Sigillaria, and the obvious affinity of its tissues to those of Conifers and Cycads, as well as the constant association with trees of this genus of the evidently phanerogamous fruits known as Trigonocarpum and Cardiocarpum. On the other hand, the resemblance to Lepidodendron may be shown to depend merely on comparisons of a part of the tissues of Sigillaria with those of that genus. Grave doubts may also be entertained as to whether strobiles of Lepidophloios, and even stems of that genus have not been improperly mixed up with Sigillaria.
It is probable that all botanists who have studied these plants, might agree that, if not Gymnosperms, they at least present points of affinity with them, and might be regarded as in some sense a link connecting them with Acrogens. Supposing this much to be admitted, important questions remain as to their possible relations to the modern Conifers and Cycads. The higher Sigillaria unquestion-
- ↑ "Conditions of Deposition of Coal," Quart. Journ. Geol. Soc. vol. xxii. p. 130.
