154 FREDERICK W. SARDESON 



differ at most in degree of accentuation. The so-called vertical 

 or median tubuli and the mesial tubuli differ most, but are com- 

 parable directly to acanthopores. Their numbers and small size 

 here characterizes the species and genus, however, and unites to 

 the Rhinidictyonidae. 



The question arises again as to what interpretation should be 

 made of the zoarium. Apparently the same interpretation is 

 necessary in Pachydictya foliataas in Trepostomata. E. O. Ulrich 

 holds that the successive loculi between autocell tabulae were 

 each a zooecium, and the autocell was built by successive genera- 

 tions of polypites and as to median and mesial tubuli, that they 

 were a united system of canals. One may prefer the other 

 interpretation for the following reasons. In any case the 

 necessary interpretation is that the skeleton or zoarium was built 

 by a cortex of zooids over its surface, since the mesopores are 

 outside the cells or zocecia, and must have been been built by 

 super-zoarial secretion ; and likewise the median tubuli which 

 end not in the cells but above them. Admitting a cortex to 

 have covered the zoarium, there is no explanation as to why 

 successive generations instead of a single polypite should have 

 built each cell, or how neighboring cells could have had, one 

 four, the other three or five generations to build it. The 

 so-called tubuli can be interpreted as the structural results of 

 surface projections which they are really seen to be. One can 

 explain the mesial lamina of the zoarium in this way, that, as 

 compared to the palmate zoarium of certain Trepostomata, with 

 long narrow axial surface and flat axial region, the bifoliate zoa- 

 rium has a still more specialized, narrower axial region and the 

 growth or axial edge being thus very narrow, the cortex bent 

 rather than curved over it, the cell walls coinciding with the line 

 of flexure coming to lie nearly in one plane, and to be more or 

 less thickened. The mesial lamina is at the axial growth center, 

 not as a germinal layer but a wall. The cell increase having 

 ceased at full maturity at any part, the margin ceased to extend 

 rapidly and a filled cell or " nonporiferous " margin formed. 



Species of Pachydictya, e.g., P. acuta Hall, might be described 



