20 FREDERICK W. SARDESON 



or shallow, relatively, in different species. The autocell walls, 

 which are thin and then more or less thickened, give respec- 

 tively polygonal or rounded calycals, since the thickening is 

 greatest at the cell angles. A beveled wall edge or impressed 

 rim around the cell aperture gives it in many a saucer-shape, 

 especially when the walls are very thick (Plate A, Fig. 11). 



Acanthopores or warts may be present on the walls, usually 

 at cell angles. A lunarium, when present, gives the cells another 

 peculiarity. The lunarial wall and the acanthopores are, how- 

 ever, mural modifications, and will be explained in that con- 

 nection later. 



Thin section is quite necessary to bring out the wall's 

 structure. The walls are dense. If thin they may show no 

 differentiation ; yet, if a specimen split, the cleavage may pass 

 longitudinally and so as to leave part of the wall attached to 

 each stone core, which fact has been taken to indicate that the 

 wall is always structurally double ! Presumably the wall is built 

 double always, i. e., the increment on the margin is continued 

 within every calycal, the wall being thus double with a median, 

 third part, which is, however, not known to be double. A thick- 

 ened wall tends to show greater differentiation, both in structure 

 and composition, than a thin one. The median wall may 

 appear either distinct or not, and correspondingly the striping 

 parallel to the wall's surface when seen, showing the laminae 

 of growth, is either interrupted by the median wall or more or 

 less distinctly crosses continuously from one side wall to the 

 other. 



To explain the structural aspect of acanthopores in thin sec- 

 tions, the following analysis may serve. A distinctly double 

 wall shows the median wall as a line in transverse section (Fig. 

 I, a) and when the wall edge is scalloped (Fig. b) the median 

 line appears interrupted in section (Fig. c), corrresponding to 

 the angles. A rounded wart (Fig. d) would produce a similar 

 effect (Fig. e) ; or, if very distinct, as in Fig./. When once 

 begun, the wart may have its own growth, so to say, independent 

 of the wall thickness (Fig. g), becoming so large as to appear 



