OF LOWER GREENSAXD PLAXTS. 



251 



(#) The excessively thickened dements are the most conspicuous 

 feature in the transverse section (Pis. XXIII, XXV, fig. 1, .), 

 and stand out as black squares owing to the nature of their 

 walls. Thej* are generally squarish or oblong (PI. XXIII, 

 fig. 2, s.) and average about 40-50 /j. in diameter, though in 

 some cases they are radially extended up to a size of 70 ^. In 

 radial direction one of these cells always alternates with a pair 

 of the thinner-walled and pitted elements ; and these series are 

 arranged so that the fibres are always laterally adjacent to 

 each other, so that they form straight tangential bands which 

 run for long distances, with no interruption save the medullary 

 ray-cells which separate adjacent elements but do not interfere 



Text-fig. 74. Vectia luccombensis, sp. nov. Transverse section of a small 

 area showing the narrow band of cork-cells, ., lying between the pair 

 of thin-walled elements, u. 1 and t\ 2 ; s., fibres. 



with their radial or tangential sequence. This can be seen very 

 well in the central part of PI. XXV, fig. 1. The radial walls 

 of two adjacent fibres are generally in complete contact (text- 

 fig. 73, s. 1 ), though in some cases these split apart, as in 

 text-fig. 73, s.-. In many places the middle lamella between 

 the cells is clearly shown, and on either side a somewhat 

 thickened white zone of the wall through which the pits can be 

 remarkably well seen (text-fig. 73, sp.) in transverse section, 

 and also in some places in longitudinal section. 



The secondary thickening of the wall is dark brown or black, 

 and therefore conceals the pits in most cases, though locally, 



