366 KIDSTON : THE FLORA OF THE CARBONIFEROUS PERIOD. 
Some Other stems, however, which have been referred to Araucari- 
oxylon (Pinites) or Dadoxylon, such as Araucarioxylon Beinertianum 
Gopp. sp. and Pitus (Araucarioxylon) antiqua Witham and others, 
in which the secondary wood agrees in all essential characters 
with that just described, possess primary as well as secondary wood. 
According to the species, the primary wood consists of few or many 
isolated groups of tracheides, usually of small extent, which are 
situated in the pith close to the surrounding zone of secondary wood, 
or resting on it. To these bundles no additions of new elements are 
made when once they are fully developed. It is probable that these 
stems, with primary and secondary wood, either in whole or in part, 
also belong to Cordaites. Many of these have been found in the 
Lower Carboniferous, especially in the Calciferous Sandstone series, 
where, however, Cordaites leaves are rare. This may be only an 
accident of circumstances, for in the beds where the large trees are 
found, almost invariably no other plant-remains are discovered with 
them. In every case which has come under my observation they 
occur as drift trees, generally embedded in sandstone or other coarse- 
grained material where delicate fossils could not be preserved. Their 
cortex, also, with one exception, has always disappeared through decay 
or attrition. It must also be further borne in mind that a natural 
sorting takes place in all water-carried vegetable material ; the 
smaller coming to rest at one place and the larger, such as tree trunks, 
at another. 
Such is the manner of distribution in which we generally find 
fossil plants to occur. 
The structure of the stems we have just been considering leads 
to very important conclusions in regard to the affinities of these 
plants. It has generally been considered, and as often stated, that 
these so-called Dadoxylon or Araucarioxylon stems belong to the 
Coniferce ; in fact, it has been usual to suppose that the Coniferce 
occupied a very important place in older palaeozoic times ; this, 
however, is not the case, for although in the structure of the secondary 
wood a great similarity is shown to that of Araucaria, still other 
more important structural characters, such as the large chambered 
pith and, when present, the mesarch structure of the primary bundles 
point much more to their affinity being with the Cycadacece. 
