64 DR. D. H. SCOTT ON A NEW TYPE OF MEDULLOSE 
stelar character than those of Wedullosa. The unilateral strands, with their external 
protoxylem, bear a considerable resemblance to those of an ordinary Myeloxylon, 
suggesting a doubt whether their concentric structure is primitive; from their 
appearance they might conceivably have been derived from bundles that were once 
collateral. This, however, is a very unlikely hypothesis, considering the gradation that 
exists from the stele to the unilateral bundles. It is much more probable, as all analogy 
indicates, that the foliar bundles of Swteliffia were on the up-grade of development, and 
that the unilateral structure of the wood may have preceded collaterality of the bundle 
as a whole. 
Taking all the characters into consideration, I should be inclined to regard Suteliffiia 
as the most primitive member of the Medullosez yet discovered; it is doubtful, 
however, whether it lay on the direct line of descent of any of the more complex types 
with which we are acquainted. It may be well to repeat here the warning already 
given (p. 54), that the adult structure of Swtcliffia itself may have been more complex 
than that shown in our solitary specimen. 
The exarch structure of the stele appears to have been common to some other Medul- 
lose, e. g., M. Leuckarti*, while M. anglica was somewhat mesarch in the stele, though 
exarch as regards the leaf-trace bundles t. The distinction is not, in any case, a very 
marked one in this group. 
Sutcliffia, if we interpret its structure rightly, had not advanced very far beyond the 
simple protostelie condition which we find represented in Heterangiwm among the 
Lyginodendres, or, in a more specialized form, in Sewards Megaloxylon. This is a 
point of some importance, as indicating that the whole course of evolution from the 
protostele to the most élaborate dialystelic type may have been gone through within the 
family Medullosez. In order to explain the origin of the peculiar Medullosean anatomy 
it is no longer necessary to go so far afield as the Botryopterideze, for example. The 
Medullose: are their own interpreters. 
The discovery of Suwtcliffie appears to add probability to the suggestion that the 
Medullosez, as well as the Lyginodendrez, may have sprung from a type anatomically 
similar, apart irom details, to Heterangiwm 1. But this must only be taken in the most 
general sense. Heterangium, with the xylem-strands of its leaf-traces already marked 
out in the stele itself, had probably advanced some little way in the direction of 
Lyginodendron, and no longer represents the common type with any exactness. So far, 
however, as the anatomical evidence indicates, the probability is that Lyginodendree 
and Medulloseze may have had a common origin from a point not very far below the 
ievel of stems such as those of Sufcliffia and Heterangiwm. On the other hand, the 
evidence for the direct derivation of the Medullosez from a simple monostelie type 
tends to remove them further from the Cladoxylez, ancient and markedly dialystelie 
forms $, which may well, as has recently been suggested, find their nearest allies among 
some of the Botryopterideze. 
x Solms-Laubach, 1897, p. 180, pl. 6. fig. 2. + Scott, 1899, p. 118. 
t Scott, 1899, p. 89, pl. 10. fig. 5. $ Solms-Laubach, 1896. 
