396 



THE WONDERS OF GEOLOGY. Lect. IV. 



The external surface is in consequence marked with lozenge- 

 shaped elevated scars, separated from each other by lateral 

 depressions, surrounded by a parallel band of a fibrous 

 structure (see Lign. 90). This outer portion is sometimes 

 consolidated into a compact cylinder of stone, which will 

 separate from the central axis.* The latter is solid, and 



strongly marked externally 

 with reticulated interrupted 

 ridges. This surface has gene- 

 rally patches of vascular tissue 

 adhering to it; and on some parts 

 of the internal axis there are deep 

 pits, indicating lacunae which 

 probably contained a resinous 

 secretion, as in the Dragon - 

 blood plant. In a waterworn 

 specimen collected from the 

 beach at Brook Point {Lign. 

 nn _ 90), the internal structure, 



Lign. 90. — Portion of the stem /' # ' 



of Clathraria lyellii ; from which is seen in thin transverse 

 brook Bay. sections, resembles that of the 



(One-third linear nat. size.) ("Veadese 



The most interesting example hitherto known was 

 obtained from a stratum of chalk marl at Bonchurch in the 

 Isle of Wight.f It consists of the summit of the stem 

 garnished with persistent petioles, as shown in the annexed 

 figure, Lign. 91. The original is fifteen inches in length ; 

 the upper part is entire, but the lower is broken, and 

 exposes the internal axis in its natural position {a), sur- 

 rounded by the false bark, formed by the consolidation of the 

 bases of the leaf stalks. The surface of the lower portion of 

 the stem is bare, and scored with the lattice-like cicatrices, 



' Fossils of Tilgate Forest, Plate II. 

 | Geology of the Isle of Wight* p. '294. 



