316 C. J. Hylander — Mid-Devonian Callixylon. 



" Seward in his great textbook (vol. Ill p. 292) says of the 

 reference of the Indiana black shale Cordaite to Callixylon — 

 'Miss Elkins and Dr. Wieland refer some upper Devonian wood 

 from Indiana characterized by a grouping of the circular or 

 elliptical bordered pits in the radial walls of the tracheids 

 similar to those in Callixylon Trifilievi, which they include 

 together with the middle Devonian species of Cordaites New- 

 berryi in Zalessky's genus. Though these two American species 

 are comparable in the discontinuous arrangement of the tracheal 

 pits with the Russian type, the latter is characterized by primary 

 xylem strands, a feature not recognized in the American stems ; 

 it would seem, then, undesirable to adopt the designation Calli- 

 xylon in preference to Dadoxylon unless there is evidence as 

 to similar characteristics in the primary xylem. ' 



Is this either a guide to convenient usage or a logical con- 

 clusion? It is not well to lay stress on the precise position in 

 the middle or upper Devonian for any of these forms, since their 

 time range is only inferable. The interesting and decisive point 

 is that in middle to later Devonian time there was a cosmopoli- 

 tan group of Cordaites with the grouped pits, and such forms 

 are known in a typical instance to have the old cryptogamic 

 wood. The probability is that all have it. But if any of the 

 species referred to Callixylon with reservations well within 

 the recognized usages of Paleobotany, were later found to lack 

 cryptogamic wood, then a new genus would be indicated, perhaps 

 a new family. Moreover, mere reference to Dadoxylon would 

 settle nothing, where these recurrently variant forms are con- 

 cerned. With or without the cryptogamic wood, there would 

 still remain the chance that the leaf or floral characters varied 

 strongly from the forms primarily designated as Dadoxylon. 

 Like so many of the genera of Paleobotany, Dadoxylon is now 

 more a group name than a genus in the purely botanical sense. 

 And similarly Callixylon, in the first instance a needed generic 

 distinction, must share the same fate of ultimate and convenient 

 inclusiveness. Only thus may we avoid the use of over-many 

 generic names in our descriptions and groupings of ancient 

 plants. The other alternative must be genera of mainly one 

 sole species. 



These more or less silicified bits of lignitic wood must have 

 attracted the attention of Marsh while searching in the lime- 

 stone which thinly covers the Hamilton, and carries both fish 

 and plant remains. They would thus be from near the close 

 of the mid-Devonian. But the horizon might be in the Hamilton 

 shale, or higher in the black shale of the Portage, also carrying 

 plants. The bits were marked by Marsh as from the Hamilton 

 group. 



