PROBLEM OF THE MONTICULIPOROIDEA I 5 I 



from whoever may be in favorable position to obtain the fossils. 

 For the present the best literature is not to be taken as alike 

 reliable on all. In extreme cases a fortuitous or abnormal 

 character in specimens of a species may have been mistaken as 

 grounds for a new species, and this being then peculiar may be 

 set up as a new genus, and in turn it affects the family. Owing 

 to the more highly specialized structures as compared to Tre- 

 postomata, Cryptostomata are, as said, somewhat easier to learn, 

 but they are more complex to study. 



For the present purpose a few common, well-known species 

 suffice. Taking the bifoliate group first : 



Pachydictya foliata Ulr. (see Plate B, Figs. 1, 2, 3) is a leaf- 

 shaped zoarium about 50 mm wide and 1 to 3 mm thick, with auto- 

 cells 0.3 to o.4 mm in diameter growing from a median plane and 

 opening on either side, "bifoliate." It grows at the margin, the 

 erect frond broadening' as it increases in height. The growth is 

 not quite uniform and a lobate, undulate shape prevails. The base 

 or broad stem of the frond is therefore the more mature, thicker 

 part, and there the cell increase ceases first and the margin 

 changes to a solid, sharp, or rounded narrow border. Growing 

 margins may also appear on the face of the frond, and when 

 large, arising near the stem, produce a so-called trifoliate frond. 

 A basal expansion incrusted the ground, and if this was broken 

 off a new one developed from above the broken edge. Any 

 injury destroying part of the frond surface gave rise to a similar 

 growth. 



The basal expansion grew perigene, the frond acrogene and 

 so that the zoarial growth and cell increase is normally at the 

 margins only. In the frond the cells arise on either side of a 

 median plane, or so-called mesial plate or lamina, are directed 

 at first vertically, but as younger cells arise between, above 

 them, they turn laterally. Thus an immature or axial and a 

 mature or peripheral region are distinguished. The mesial lamina 

 is built as the extreme wall at the growing edge of the frond. 



The cells at the beginning, i. e. at the mesial lamina, are thin 

 walled, with mesopores at their angles, but later they are separated 



