302 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
Mr. Thomas Brittain, of Manchester, for sections of the specimen discovered by 
Mr. Butte rwortii. 
Index to the Plates. 
PLATE 47. 
Lepidoclendron setaginoides. 
Fig. 1. Transverse section of a young twig of Lepidodendron selaginoides, enlarged 
13 diameters, a. Vasculo-medullary axis. b. Innermost cortex, d. Zone 
from which the middle bark has disappeared, e. Parenchymatous portion 
of the outer bark, passing into the bast-layer f g. Leaves. 
PLATE 51. 
Fig. 2. Segment of a similar section to fig. 1. Preference letters as in that figure. 
O O O O 
Enlarged 27 diameters. 
• PLATE 48. 
Transverse section of the centre of a twig in which an exogenous vascular zone, 
h, starting from opposite the point x, has extended both radially and laterally. 
c, c', foliar vascular bundles in the innermost bark. Enlarged 36 diameters. 
Segment of a similar section with the exogenous zone further advanced. 
Enlarged 12 diameters, i, i. Accidental fissures in the bark. 
PLATE 49. 
Fig. 5. Vasculo-medullary axis and exogenous zone of a section similar to fig. 4. 
Enlarged 28 diameters. 
PLATE 50. 
Fig. 6. Central axis of a more advanced twig in which the exogenous growth, h, has 
completely enclosed the vasculo-medullary axis. Enlarged 27 diameters. 
Fig. 7. Transverse section of a branch of Lepidodendron selaginoides of larger size, 
and which has reached a perfectly Diploxyloid stage of growth, e. Pa¬ 
renchymatous zone of outer bark, f Bast layer of outer bark. Enlarged 
nearly 6 diameters. 
Fig. 3. 
Fig. 4. 
