TJie Fossil Plants of the Coal Measures. 119 



the medullary cells ; a very distinct feature, which, with 

 this exception, I have only seen in L. selaginoides. On the 

 other hand, it resembles L. fuliginosum in the small size, 

 and the tortuous and irregular vertical arrangement of the 

 tracheae of its secondary xylem strand. This combination 

 of the characteristics of two such extremely distinct types 

 makes this plant a very interesting one. Its cortex 

 approaches nearest to that of L. selaginoides. The diameter 

 of the perfect twig is about "5. 



Tra7isverse. 



Q. — p. 198, Fig. 16, C.N. 417 ; see also 418. 



Longitudinal. 



Q.— p. 198, Fig. 17, C.N. 419. 



Type of Lepidodendron parvidiini. Will. 



This type is only known in the form of slender twigs, 

 which rarely exceed 'i of an inch in diameter. Its principal 

 characteristic occurs in its cortical tissues. Though so small, 

 it has a distinct medulla, surrounded by the usual primary 

 tracheal cylinder. The innermost cortex has disappeared 

 from all my specimens ; but a narrow cylinder of a middle 

 cortex is always preserved, as is also the external one, with 

 its periphery of leaves. The inner border of this outer 

 cortex is characterised by forming a broad zone of circular 

 loops or arches, the bases of the radiating pillars of which 

 blend with the middle cortical ring. When perfect, each of 

 these arched areas is filled with a distinct thin-walled 

 parenchyma interposed between the middle and the outer 

 layers of the bark. In one series of six sections of the same 

 branching twig, we trace the usual dichotomy of the primary 

 tracheal cylinder from its normal condition of a single ring 

 through the various stages of development, first into two 

 semicircular halves, up to the point where each half has 

 developed into the perfect primary tracheal cylinder of a 

 separate branch. 



