Derby — Crown Structure of Psaronius Braziliensis. 153 



cross sections proves that they soon ceased to grow. The 

 younger pair (C3, C4) present no essential modifications, while 

 the central members of the C group show changes in shape, 

 size and position which possibly may be due to the substitution, 

 by new sprouts, of old members that ceased to grow in the 

 interval between one cut and the other. 



Fig. Ill is plotted on the upper face of the Rio slice as a 

 base and rises to the top of the Paris one. The nearly mature 

 F organs are cut at the beginning of the lateral emergency of 

 the front one and somewhat below the top of the persistent 

 stub of the back one. The circumstance that Fo did not reach 

 the level of the upper face of the Paris slice shows that it was 

 slightly in advance (10-20 mm ?) of its fellow. Its condition 

 a little before the level of the front of the cut was attained 

 must have been similar to that of F4 in fig. II and its 

 position was just within the stem rind but not quite flush with 

 it. Its slightly rounded front face seems to have commenced 

 to bulge outward over the rim of the stem rind, interrupting 

 the upward growth of the latter, and its lateral sclerotized 

 layers, being brought in contact with the stem rind, commenced 

 to fuse with it. Between this level and that of the lower face 

 of the Paris slice but at a point too high to be included in the 

 cut, the central part of the organ seems to have grown away 

 from the sclerotized layers and from the central parenchyma, 

 leaving the former behind and fully incorporated with the stem 

 rind but with an interval between their inner margins which soon 

 became sclerotized (see Strasbourg to London 2 cross sections 

 in lig. I ; op. cit.) so as to make the stem rind complete 

 behind the organ which still continued to grow upward 

 parallel to the stem before bending outward and assuming 

 characters that have not been preserved in the fossil. In this 

 same interval of growth (see lower face of Paris slice in fig. I, 

 op. cit.) the vascular strand became greatly modified through 

 the dropping out of its broad central portion, leaving two 

 irregular lateral strands that doubtless assumed the form 

 shown by Fl and F2 in fig. I and eventually, just before 

 bending outward, that of F4 in fig. III. 



The P organs, which as stated above are assumed to be a 

 new set and are accordingly lettered PI', etc., are still some- 

 what abnormal at the base of this segment, as is shown by the 

 front ones, but at the higher level all four of them had become 

 normal, as is shown by those at the back of the cut. Com- 

 pared with the corresponding sections in fig. 1, a very close 

 general agreement is noted, but with a reversal of the position 

 of the close and open inrolled margin. This reversal is evi- 

 dently due to the position of these organs relative to the scle- 

 rotized face of the neighboring F ones. The small organs 



