258 PTERIDOPHYTA. [CH. 



partly with a view to illustrate the geological history of the 

 genus, and partly to contribute something towards a fuller 

 knowledge of particular species. One of the most striking facts 

 to be gleaned from a general survey of the past history of 

 the Equisetaceae is the persistence since the latter part of the 

 Palaeozoic period of that type of plant which is represented by 

 existing Equisetums. There is perhaps no genus in existence 

 which illustrates more vividly than Equisetum the survival of 

 an extremely ancient group, which is represented to-day by 

 numerous and widely spread species. The Equisetaceous 

 characteristics mark an isolated division of existing Vascular 

 Cryptogams, and without reference to extinct types it is 

 practically impossible to do more than vaguely guess at the 

 genealogical connections of the family. When we go back to 

 Palaeozoic plants there are indications of guiding lines which 

 point the way to connecting branches between the older Equi- 

 setales and other classes of Pteridophytes. The recently 

 discovered genus CJieirostrohus'^ is especially important from 

 this point of view. 



The accurate description of species, and the determination 

 of the value of such ditferences as are exhibited in the surface 

 characters of structureless casts, are practically impossible in 

 many of the fossil forms. In certain living Horse-tails we find 

 striking differences between fertile and sterile shoots, and 

 between branches of different orders. The isolated occurrence 

 of fragments of fossil stems often leads to an artificial sepa- 

 ration of ' species ' largely founded on differences in diameter, 

 or on slight variations in the form of the leaf-sheaths. It is 

 wiser to admit that in many cases we are without the means of 

 accurate diagnosis, and that the specific names applied to fossil 

 Equisetums do not always possess much value as criteria of 

 taxonomic differences. 



The specimens of fossil Equisetums are usually readily recog- 

 nised by the coherent leaf-segments in the form of nodal sheaths 

 resembling those of recent species. The tissues of the cortex 

 and central cylinder are occasionally represented by a thin layer 



^ Scott (97). This genus will be described in Volume ii. 



