HETERANGIUM 415 



intervals of a couple of centimetres or more, large stalked 

 leaves, with a repeatedly branched rachis, bearing numer- 

 ous, finely segmented leaflets ; the arrangement of the 

 leaves on the stem was a spiral one, and the commonest 

 divergence appears to have been f . Now there, is a" 

 " Fern," of which the external characters are well 

 known, which agrees in all the points mentioned with 

 our fossil, and which is common in deposits of the 

 same geological horizon, both in Scotland and on the 

 Continent. This is the Sphenopteris elegans of Brong- 

 niart, which is represented in Figs. 159 and 160, from 

 Stur's figures. The stem, a portion of which, slightly 

 reduced, is shown in Fig. 159, bears the bases of 

 several petioles, and Stur, who investigated specimens 

 from the Culm of Bohemia, found that the phyllotaxis 

 was §. The angles of the stem correspond to the 

 orthostichies of leaves ; the general aspect of the frond, 

 which was large and of the Diplotmema form (i.e. with 

 the primary pinnae forked), is indicated in Fig. 160. 



But, besides the general resemblance in external 

 habit, there is one character which appears decisive 

 as to the identity of Heterangium Grievii with Spheno- 

 pteris elegans. It will be remembered that the inner 

 cortex of H. Grievii, both in stem and petiole, contains 

 numerous horizontal plates of stone-cells, giving a 

 characteristic transverse striation to the specimens, 

 wherever this part of the tissue is exposed to view. 

 Now in Sphenopteris elegans, though only preserved 

 in a carbonised condition, without any minute structure, 

 precisely the same feature is shown ; wherever the super- 

 ficial layers had broken away from the stem or leaf- 

 stalk, so as to expose the more internal cortical tissues, 



