The plants of the Palaeozoic period, though often differing considerably from those of the same class in the floras of to-day, exhibit a remarkably high type of organisation. Some of the most abundant trees in the forest of the Coal age are decidedly superior in the complexity of their structure, as also in size, to modern survivals of the same stock. On the other hand, it must be remembered that Monocotyledons and Dicotyledons which now occupy the highest place in the hierarchy of plants have left no sign of their existence in any of the Palaeozoic strata. The greater size of some of the Palaeozoic plants, and in some respects the more advanced stage of evolution which they represent as compared with their nearest relatives of the present era, must be considered in relation to their more important and relatively higher position in the plant-world than that which is now held by their diminutive descendants. It is, however, impossible to get away from the conclusion that the oldest Palaeozoic flora of which we have an intimate knowledge must be the product of development of an age which is represented by a chapter in the history of the plant kingdom at least as far removed from the beginning as it is separated from the chapter now being written. Examples might be quoted in illustration of the risks attending the determination of fossils by means of external features alone, but it may suffice to mention the case of a specimen originally described as a fragment of a Cretaceous Dinosaur under the name _Aachenosaurus multidens_. By the examination of thin sections this supposed bone was shown to be a piece of Dicotyledonous wood(42). The methods of preservation of plants as fossils are numerous and varied and the few examples selected give but an incomplete idea of the subject: for a fuller treatment of fossilisation the reader is referred to more technical treatises (48 vol. I.).