BORDERED PITS 



61 



four-seriate in C. Clarkei. In the majority of species the rows are 

 not constant, but show a varying number from one to three, or 

 two to five, this variation being exhibited by adjacent tracheids 

 in accordance with the variation of the latter in radial diameter; 

 and viewing this distribution as a whole, it cannot be doubted 

 that it represents corresponding differences in development. One 

 of the most striking features of the genus is to be met with in 

 C. Newberryi (plate 8), which is unique in the segregation of 

 the pits into groups of six to thirteen. 



In Araucarioxylon (28, 614), while conforming to the char- 

 acteristic form and compact arrangement presented in Cordaites, 

 the pits exhibit far less constancy in their serial arrangement, 

 and in this respect they are at once comparable with those of the 

 existing Araucarias. Among the latter A. Cunninghamii shows a 

 one- to three-seriate disposition, A. excelsa is one- to two-seriate, 

 while A. Bidwillii is strictly one-seriate. All of the extinct species 

 as comprised in the genus Araucarioxylon not only show similar 

 variations, but such variations are found to cover a much wider 

 range. A comparison of all the species, both recent and extinct, 

 now available for that purpose is of interest in this connection. 



Existing species. 



