182 SECTIONAL ADDRESSES. 
6. The Protopitys type, a singularly isolated one, elucidated by 
Solms-Laubach (Lower Carboniferous). The above are all monostelic. 
Next come the two essentially polystelic groups: 
7. Cladoxyleze, already mentioned, a somewhat mysterious race, of 
Lower Carboniferous or possibly even Upper Devonian age. 
8. The well-known Medullosezs (Upper Carboniferous). 
It is noticeable that five of these families are Lower Carboniferous 
(or possibly, in certain instances, older); one (Lyginopteridess) includes 
both Lower and Upper Carboniferous members, while two (Megaloxylez 
and Medullosee) are at present known only from the Upper 
Carboniferous. 
Of the eight families in question there are only two (Lyginopteridez 
and Medullosez), in which we have any evidence as to the fructification. 
The other six are known only by their vegetative and mostly by their 
anatomical features. Of these the Protopityee and the Cladoxylez 
are the most isolated, differing, for example, in the structure of their 
tracheides from the other families. There seems to be no reasonable 
doubt that the families represented by Lyginopteris, Rhetinangium, 
Megaloxylon, Calamopitys, Stenomyelon, and Medullosa are related, 
and belong to one and the same main phylum. Considering that 
members of two widely separated families in this series are known 
to have borne highly organised seeds, there is a strong presumption 
that the whole set were reproduced by seeds of some sort. In the 
case of the two families Protopityee and Cladoxylee the marks of 
affinity are less obvious, but even here there is rmore in common with 
the type-families Lyginopteridee and Medullosesw than with any other 
group. 
I think then that we are justified, in the present very imperfect state 
of our knowledge, in provisionally keeping all these families together, 
as probably, in some wide sense, Pteridosperms. On this view, they 
formed a distinct, extensive, and varied class of plants, already very well 
developed in Lower Carboniferous times, and no doubt going back to 
the Upper Devonian, though here the available evidence is scanty. 
The question may be asked: Did all the Seed-plants pass through 
the Pteridosperm phase, or were there other parallel lines of descent? 
Some recent work, no doubt, tends to link up the Cordaitales with the 
Pteridosperms. Mesozrylon, for example, is merely a Cordaites with 
centripetal wood in the stem, a character which strongly suggests an 
affinity with the Lyginopteris or Calamopitys type. In fact, some 
members of the Calamopityeze (Zalessky’s Hristophyton) show a certain 
approach to Cordaitales. 
A more striking point is that no marked distinction has been found 
between the seeds of Pteridosperms and those of Cordaitales. The 
general community of seed-structure is strong evidence of close affinity 
and of a common stock. 
There seems to be no proof that the family Cordaiteze existed as 
such in Devonian times; we do not know much about them even 
in the Lower Carboniferous ; the family is typically Upper Carboniferous 
and Permian. On the other hand, the Pitys family, which we include 
in the wider group Cordaitales, is as old as any known Pteridosperm ; 
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