372 ME. AETHUR JOHN MASLEN ON THE 



Comparinj^ this form, as illustrated by fig. 21, with the forms already described, it 

 will be seen that the xylem cylinder x. is thin and the pith-cavity relatively large, points 

 in which it resembles my a form, although the number of larger xylem elements in 

 the radial direction is somewhat greater. The medulla can be seen in one of the 

 longitudinal sections (S. 77) to consist of square-ended cells arranged in vertical rows 

 and somewhat elongated in the vertical direction. 



No trace of the inner cortex can be seen in fig. 21, but this is probably an accident 

 of non-preservation consequent on the crushing to which the cones had been subjected 

 previous to and during fossilization. 



Comparison of this figure with PL XXXVI. fig. 1 will serve to bring out other 

 difl'erences. It will be seen that in the latter form the outer cortex is much thicker, 

 and hence more resistant, a fact w hich may possibly account for the flattened form 

 of the specimen shown in fig. 21. Another point of difference is to be seen in the 

 much larger number of leaf-trace bundles cut across in the transverse section of the 

 7 form. In C.X. 1613 d, S. 85 (PI. XXXVI. fig. 1), and C.N. 568 (PI. XXXVII. 

 fig. 23) the number of such bundles (including those in the outer cortex) is about 

 30, whereas in S. 610 (PI. XXXVII. fig. 21) the number reaches about 60. 



The leaf-trace bundles are also very small, having a diameter (including the inner 

 cortical sheath) of but little over "1 millim. ; whereas those of C.N. 568 (PI. XXXVII. 

 fig. 23, my /3 form) are quite twice that size. Tliere is, however, closer agreement in 

 size when compared witli the a form, in which the leaf-trace bundles have a diameter 

 of about '15 millim. 



On the whole this form agrees, in the diameter of the whole cone, the diameter of the 

 axis of the cone and of the leaf-trace bundles, the thickness of the xylem cylinder, the 

 large pith, &c., more closely with my a type, from which it differs, however, in the 

 thinness of the outer cortex, the large number of leaf-trace bundles, the small diameter 

 of the vascular cylinder, &c. As compared with my /S type the differences are still 

 more accentuated: the thickness of the vascular cylinder is much less, the pith is 

 relatively much larger, the cone itself is smaller, while the leaf-trace bundles are only 

 half the size but double the number. 



It is interesting to find in the Lepidostrobi much the same series of graduations 

 between forms in which ihe vascular cylinder is very thin and those in which the 

 vascular axis is nearly solid, i. e. without pith, as Williamson described in the vegetative 

 axes of Lepidodendron. Starting from forms such as that represented on PI. XXXVI. 

 figs. 1 & 2, in w^hich the vascular ring is very thin indeed, w^e pass through forms like 

 PI. XXXVII. fig. 21, in which the number of elements in the radial direction is some- 

 what greater, to PI. XXXVII. fig. 23, in which the pith is much reduced in size, 

 finally reaching the forms described by Williamson as Lepidostrobus Traquairia, in 

 which the pith consists of but a few cells only*. 



With regard to this latter form {Z. Traquairia, Will.), which, from association, 

 Prof, Williamson was led to believe is the strobilus of which the curious objects known 



* See Williamson, '' Organization," Part X., Phil. Trans. 1880, pi. 21. fig. 82. 



