158 FREDERICK IV. SARDESON 



resrion is four instead of two cells thick. As the cells turn into 

 the peripheral region the walls thicken and distinct mesopores 

 intercalate. The autocell has a saucer-shaped calycal at the sur- 

 face and at its center a narrow cell-opening. The small cells or 

 mesopores are either similar, especially in young fronds, or again 

 have the cell opening and even the calycal filled solid, in which 

 case the autocell calycals also become ill-defined, and the sur- 

 face feature is that of small cell openings and broad rounded 

 interspaces. This condition is the maturer stage. The meso- 

 pores are either few and the autocells subquadrangular or polyg- 

 onal, or again mesopores are very numerous and autocells 

 rounded. Large or small groups of mesopores forming maculae 

 occur in broader parts of the branches, and the o.5 mm wide 

 border has mesopores only. A constricted, closed fenestrule 

 simulates therefore a macula. 



The mesopores frequently differ in size and some of them 

 appear to become autocells, peripheral cell increase thus exist- 

 ing, although necessarily to a limited degree, since the peripheral 

 cells' length is only about equal their diameter. 



Thin sections reveal no tabulae in either autocell or meso- 

 pore. The thick walls are dense with a striping parallel to the 

 surface calycal bottoms, i. e., converging upwards to median wall. 

 The mesopore openings are seen to constrict and close while 

 their calycals fill, and then the increments of growth or striping 

 cross them continuously, direct, or arched upward instead of 

 downward across the mesopore space. In this case the amalga- 

 mation may be so complete that the appearance is that of one 

 wall between autocell openings. This filling of the mesopore 

 space contrasts with that of Pacliydictya in that it is by laterally 

 thickened vertical walls instead of by vertically thickened trans- 

 verse tabulae. Notably the results are very similar in the end. 



In closed mesopore stage the zoarial margins become solid, 

 smooth, or striated, and in the very few zoarial basal stalks seen 

 the autocells also are filled on the surface above the beveled 

 zoarial articulation. Ulrich describes moreover certain closing 

 apercula over cell apertures at the bottoms of calycals, but their 



