518 STUDIES IN FOSSIL BOTANY 



lower level the centripetal wood appears to die out, as 

 in Poroxylon or Calamopitys Beinertiana. 



The interest of Dadoxylon Spenceri lies in its com- 

 bining the wood-structure of a typical Cordaites with 

 the primary xylem-strands of Lyginodendron or Calamo- 

 pitys in a reduced form. 



The exact relations of Pitys and Dadoxylon Spenceri 

 cannot be determined until we know more of their 

 structure. The characters already investigated indicate 

 that these plants, like the Poroxyleae, belong to a 

 plexus of forms connecting the Pteridosperms and 

 Cycadofilices with the typically Gymnospermous family 

 of the Cordaiteae, but standing nearer to the latter. 



III. The Cordaiteae 



The forms hitherto considered in this chapter appear 

 in some respects intermediate between the Pteridosperms 

 and the true Gymnosperms ; we now pass on to a 

 family which belongs beyond question to the latter 

 class. The Poroxyleae and Pityeae are at present 

 chiefly or solely known to us by their vegetative 

 characters ; we have now to deal with a group in 

 which our evidence is drawn from all the organs of 

 the plant— reproductive as well as vegetative. Though 

 even here our knowledge is but limited and urgently 

 needs to be extended, the fact remains that the family 

 in question — that of the Cordaiteae — is one of the 

 best known, as it is also among the most remarkable, of 

 those which fossil botany has revealed to us. 



Among the vegetable remains from the Carboni- 

 ferous and Devonian strata, specimens have long been 



