IMTERNAL STRCTURE OF TRUNKS. 373 



trunk and scales, in our search for their distinguishing cha- 

 racters. 



I have elsewhere (Geol. Trans. London, N. S. vol. ii. part 

 iii. 1828) instituted a comparison between the internal struc- 

 ture of two species of these fossil trunks, and that of the 

 trunks of a recent Zamia and recent Cycas.* 



I must refer to the memoir, in which these sections are 

 described, for specific details as to the varied proportions 

 and numerical distribution of these concentric circles of 

 laminated wood and cellular tissue, in the trunks of living 

 and fossil species of Cycadese.f 



* M. Ad. Brongniart has referred these two fossil species to a new genus, 

 by the name of Mantellia nidiformis and Mantellia cylindrica; in my paper 

 just quoted, I applied to them the provisional name of Cycadeoidea megalo- 

 phylla and Cydadeoidea microphylla; but Mr. Brown is of opinion, that 

 until sufficient reasons are assigned for separating them from the genus 

 Cycas or Zaraia, the provisional name of Cycadites is more appropriate, as 

 expressing the present state of our knowledge upon this subject. The name 

 Mantellia is already applied by Parkinson (Introduction to Fossil Org. 

 Rem. p. 53) to a genus of Zoophytes, which is figured in Goldfuss, T. vi. p. 

 14. 



t Plates 60, Fig. 1, and Gl, Fig. 1, represent very perfect specimens of 

 fossil Cycadites from Portland, now in the Oxford Museum; both having 

 the important character of Buds protruding from the Axillae of the leaf 

 stalks. 



The section given in PI. 59, Fig. 2, of the trunk of a recent Zamia hor- 

 rida, from the Cape of Good Hope, displays a structure similar to that in 

 the section of the fossil Cycadites megalophyllus from the Isle of Portland ; 

 (PI, 60, Fig. 2) each presents a single circle of radiating lamiuEB of woody 

 fibre, B, placed between a central mass of cellular tissue, A, and an ex- 

 terior circle of the same tissue, C. Around the trunk, thus constituted 

 of three parts, is placed a case or false bark, D, composed of the persis- 

 tent bases of fallen leaves, and of abortive scales. The continuation of the 

 same structure is seen at the summit of the stem, PI. 60, Fig. 1, A. B. C. D. 



The Cycadites microphyllus, PI. 61, Fig. 1, affords a similar approach 

 to the internal structure of the stem in the recent Cycas. The summit of 

 this fossil exhibits a central mass of cellular tissue (A,) surrounded by 

 two circles of radiating woody plates, B. b., between these laminated 

 circles, is a narrow circle of cellular tissue, whilst a broader circle of 

 similar cellular tissue (C) is placed between the exterior laminated circle, 

 (b) and the leaf scales (D.) This alternation of radiating circles of wood 



VOL. I.— 32 



