PROBLEM OF THE MONTICULIPOROIDEA 



21 



to have displaced a young cell or mesopore. Such a large 

 acanthopore inflects the cell wall, forming a vertical rib or 

 pseudoseptum as a rule. No doubt the acanthopore end, or wart 

 on the wall, extended to fill an invagination in the web or cortex 

 which bound the zooids to which the cells belonged. Explana- 

 tion of the cause of such invagination need not be attempted 

 here. But it may be added that the walls were evidently built 

 by surface secretion, and that the growth of a projecting wart 

 would be accumulative as compared to a plane surface, other 

 things being equal. This may explain why acanthopores are 



# 



4 



Fig. i. 



often so independently large. A scalloped or saw-edged 

 wall would not develop so. The transverse of a wall would, 

 however, if the zooids and cortex did not draw upwards fast 

 enough to prevent it. Thus a wart thickens in two dimensions 

 to the wall's one. Since, however, the wart is on the wall, it 

 receives below the wall thickness also, and hence acanthopores 

 might well be four or more times the thickness of the wall from 

 result of secretion alone. In fact, it is necessary to explain why 

 the walls, etc., can be very thin, which is evidently because the 

 zooids drew forward rapidly. 



It is fair to presume that structural differentiation of the 

 wall may be accompanied by difference in composition ; and that 

 may explain why the elevated lunarial wall may be different 

 colored and textured than the main wall ; or, again, lucid vertical 

 lines or "lunarial tubuli " only are different, these being the 

 vertical extensions of tooth-like elevations on the lunarial wall 

 margin. Acanthopores show similar differences. Such structures 

 might be mistaken for mural pores in thin sections. Mural 

 pores do not occur. 



