255 
Text-fig. 2. Transverse section of part of stele, showing a strand of spirally thickened 
tracheides of the primary wood, x 70. xy . 1 — primary xylem ; xy ? — secondary xylem ; pxy. = 
strand of spirally marked tracheides. 
Text-fig. 2). These groups evidently indicate the position of the protoxylem 
elements, but from transverse sections it is impossible to say with certainty 
whether the development of the protoxylem was strictly exarch or slightly 
mesarch in nature, and the longitudinal sections did not enable the point to 
be decided. The structure of a stele is strikingly similar to that of a stele 
of Medullosa pusilla , in which also the position of the protoxylem group 
could not be determined with certainty. In the star-ring the strand of 
spiral elements is represented by three or four tracheides only (as seen in 
oblique section), and the development of the protoxylem appears to be 
almost certainly exarch. It is, therefore, probably exarch in the outer ring 
Medullosa from the Lower Coal Measures. 
portions, and in the arrangement, of the two constituents than is the case 
with M. anglica . The main bulk of the elements of the primary wood con- 
sists of multiseriately pitted tracheides which pass over almost insensibly into 
the similar, though somewhat smaller, elements of the secondary xylem. 
Here and there towards the periphery of the primary wood occur large 
strands of tracheides, considerably smaller in size than the ordinary primary 
xylem elements, and showing spiral thickenings on their walls (pxy., 
