GENERAL AND COMPARATIVE STRUCTURE. 41 



any trace of their originally duplex nature. In all cases, more- 

 over, whatever may be the structure of the walls of the tubes 

 in their final and most developed condition, the corallites co7n- 

 mence with thin and apparently indivisible walls. Hence, in the 

 axial and deeper regions of any Monticuliporoid, long sections 

 always exhibit the thin and seemingly structureless walls which 

 exist throughout the entire length of the tubes in forms like 

 M. petropolitana, Pand. ; and this feature is equally shown by 

 transverse sections. 



In very many Monticuliporoids, however, the walls of the 

 corallites in their peripheral and more superficial portions 

 become more or less extensively thickened by a secondary 

 deposit of sclerenchyma. In some of these cases — as, for 

 example, in M. pulchella, E. and H, — the original walls of the 

 tubes can still be clearly recognised, traversing the centre of 

 the light-coloured secondary sclerenchyma as so many thin 

 dark lines. In other cases, as in forms like M. ttuuida, Phill., 

 M. Ulrichii, Nich. (fig. 3, a), M. inoniliformis, Nich. (fig. 3, b), 

 and many others, the original lines of demarcation between 

 adjoining tubes can be only indistinctly or not at all made out, 

 and the corallites seem to be indistinguishably amalgamated. 

 In such cases, lonoritudinal sections o'ive us a clearer idea of 

 the mode of formation of the thickened wall than we obtain 

 from tangential sections. In such cases, namely, the longitud- 

 inal section of the wall (fig. 3, a and b) shows that it is com- 

 posed of a succession of superimposed conical layers of scleren- 

 chyma, which are deposited one above the other as the growing 

 margin of the wall is carried upwards. The growth of the 

 thickened wall is not, therefore, effected by the deposition of a 

 new layer of sclerenchyma along the entire interior aspect of 

 the already existing tube-cavity, but is carried on simply by the 

 successive formation of new laminae of calcareous matter at the 

 margin of the old calice. The structure of the wall, therefore, 

 is in these cases precisely similar to what is found in Steno- 

 pora, Lonsd., except that the production of new layers of scler- 

 enchyma appears to take place continuously and regularly, 



