150 THE GEOLOGICAL HISTORY OF PLANTS. 



separate and in transverse lines. Witham's name was changed by 

 Goeppert to Araucarites, to indicate the similarity of these woods to 

 Araucaria, Pinites being reserved for trees more closely allied to the 

 ordinary pines. Endlicher, restricting Araucarites to foliage, etc., 

 of Araucaria-like trees, gave the name Dadoxylon to the wood ; and 

 this, through Unger's " Genera and Species," has gained somewhat 

 general acceptance. Endlicher also gave the name Pissadendron to 

 the species which Witham had called Pi tus ; but Brongniart pro- 

 posed the name Palceoxylon to include all the species with thick 

 and complex medullary rays, whatever the arrangement of the discs. 

 In Schimper's new work Kraus substitutes Araucarioxylon for End- 

 licher's Dadoxylon, and includes under Pissadendron all the species 

 placed by Brongniart in Palceoxylon. 



To understand all this confusion, it may be observed that the 

 characters available in the determination of Palaeozoic coniferous 

 wood are chiefly the form and arrangement of the wood-cells, the 

 character of the bordered pores or discs of their walls, and the form 

 and composition of the medullary rays. 



The character on which Witham separated his genus Pities from 

 Pinites is, as I have ascertained by examination of slices of one of 

 his original specimens kindly presented to me by Mr. Sanderson, of 

 Edinburgh, dependent on state of preservation, the imperfectly pre- 

 served discs or areolations of the walls of the fibre presenting the 

 appearance of separate and distinct circles, while in other parts of 

 the same specimens these discs are seen to be contiguous and to as- 

 sume hexagonal forms, so that in this respect they do not really 

 differ from the ordinary species of Dadoxylon. The true character 

 for subdividing those species which are especially characteristic of 

 the Carboniferous, is the composite structure of the medullary rays, 

 which are thick and composed of several radial piles of cells placed 

 side by side. This was the character employed by Brongniart in 

 separating the genus Palceoxylon, though he might with convenience 

 have retained Witham's name, merely transferring to the genus the 

 species of Witham's Pinites which have complex medullary rays. 

 The Brian rocks present the greatest variety of types, and Palceoxylon 

 is especially characteristic of the Lower Carboniferous, while species 

 of Dadoxylon with two rows of bordered pores and simple medullary 

 rays are especially plentiful in the upper coal-formation and Permo- 

 Carboniferous. 



The following table will clearly show the distinctive characters 

 and relations of the genera in question, as held by the several authors 

 above referred to : 



