of the Fossil Plants of the genus SigUlaria, 275 



general are very distinct, and the lower part of the leaf-base of one 

 spiral is rarely carried over more than the right half of the superior mar- 

 gin of the leaf-base belonging to the spiral immediately to the left ; but, 

 in the example under consideration, the inferior fart of one leaf-hate is 

 often nearly carried over the whole of the superior margin of another, and both 

 have become so closely united, that the lines of junction are, in many instances, 

 nearly obliterated. As regards the threads of cuticle, they are, in many 

 places, conjluent with the leaf-bases on the left, and their connection with these 

 parts, that is, when they unite two different ribs, is often broken in their passage 

 across the furrows. These circumstances, it is evident, have given rise to the 

 decided form which the ribs of this plant have assumed. Further, they 

 throw considerable light, in my opinion, on the cause of the ribbed ap- 

 pearance of Sigillaria ; for if the leaves of this fossil are resolvable into 

 spirals similar to those which have been traced on ConifercB and Lepido- 

 dendra, then nothing more is necessary for the production of its ribs, 

 than the completion of what is only partially effected in Abies Webbiana, 

 Now, let us, in the first place, endeavour to ascertain if such a spiral 

 arrangement of the leaves has obtained on the stems of Sigillaria. 

 On the specimen outlined in figure 6 (PL V.), the leaf-scars, although 

 arranged in longitudinal series, nevertheless agree with the lozenges of 

 Lepidodendron, and the leaf-bases of Coniftrce, in following a spiral course. 

 Judging of the means by which the direction of the spiral has been 

 traced in these plants, the inquiry suggests itself, as to whether any spe- 

 cimens of Sigillaria shew appearances of threads of cuticle. As yet, 

 I have not observed any positive evidence of their occurrence on any of 

 the examples which have come under my notice. In the absence of this 

 kind of evidence, our next way of proceeding will be, to see if the leaf- 

 scars of one rib may be connected with those of another, so as to har- 

 monize with the results that have already been obtained as regards 

 other plants. In consequence of the leaf-base c x, being below the leaf- 

 scar b, it is manifestly impossible that they can be a continuation of 

 each other, or pertain to one series ; they are precisely in the case of 

 the lozenges a and b on the specimen of Lepidodendron, represented in 

 figure 4, which lozenges, it will be perceived, were shewn to belong to 

 two distinct spirals ; but there is no difiiculty in the way of the leaf- 

 bases lettered b x, being consecutive to each other. By assuming that 

 the latter are in the direction of a spiral, it will be seen that we have 

 exactly the counterpart of what prevails in Lepidodendron, when the 

 stems of this fossil have not undergone any unusual development in 

 the direction of their axis, viz., two leaf-bases (c x and a x) of two 

 spirals intervene those (6 x and b x) which belong to the spiral, whose 

 course has been assumed, and, confining ourselves to the former, the left 

 one is lower than that on the right. Further, let us, in the second place, 

 endeavour to ascertain how far the circumstances which have produced 

 the ribs of Abies Webbiana, have contributed to form the like character 

 on Sigillaria. Referring to the figure of the Nepaul pine, it will be seen. 



