3_a_.--Transverse section of calcareous coniferous wood, from Willingdon, Sussex, × 80 linear; p. 173. 3_b_.--Longitudinal section of the above, × 120 linear. 4.--Slice of a transverse section of a recent Dicotyledonous Stem; showing, 1st, Pith or medullary column, occupying the centre; 2d, Four bands of woody layers, separated by condensed lines of elongated tissue in series, and having large regular openings of vessels, with numerous medullary rays running continuously from the central pith to the bark; 3d, the bark. (_From Mr. Witham._) 5.--Slice of a transverse section of a recent gymnospermous phanerogamic stem (of a Cycas), having a central pith, with woody layers separated by a condensed line, and consisting of elongated cellular tissue, arranged in a regular series; medullary rays and bark. (_From Mr. Witham._) 6.--Bundles of vascular tissue in Stigmaria ficoides, × 12 linear. See p. 135. The two strands of vessels that appear as if on the surface (and are of a looser texture) are part of the vascular tissue of the stem, and become inflected (that is, bent over), and give rise to a band of vessels (the darker band seen between the above), that passes towards the bark or cortical covering. 7.--Portion of a transverse section of one of the bundles of vascular tissue of _Sigillaria elegans_, × 20 linear. (From _M. Brongniart._) See p. 131. The convex portion on the left, and which in the original stem is situated towards the centre, is composed of the medullary vascular tissue formed of vessels irregularly disposed. The longitudinal bundles are the woody fibres arranged in a radiated circle: the smooth interspaces are medullary rays. The two distinct roundish spots of vascular tissue on the right of the ligneous zone occur irregularly on the outside of the woody circle, and are supposed to be detached bundles of the ligneous zone extending towards the leaves. See p. 131. [Illustration: _Pl. 6._ _J. Dinkel del. G. Scharf lithog._ _Printed by Hullmandel & Walton_] DESCRIPTION OF PLATE VI. _Illustrative of the Structure of Fossil Teeth._ Fig. 1_a_.--Tooth of _Psammodus porosus_, from the Mountain Limestone. See p. 587. 1_b_.--Vertical section, a portion × 75 linear. 1_c_.--Transverse section of the same, × 75. 2_a_.--Tooth of _Ptychodus polygurus_, from the Chalk, near Lewes. See p. 585. 2_b_.--Portion of longitudinal section, × 20. 2_c_.--Portion of transverse section, × 20.