WALL STRUCTURE o61 



pearances as are shown in figure 4:~) might perfectly well be interpreted 

 as indicating a dniible wall, were it not for the fact that this pliase and 

 the phase shoM'n in figure 43 may and do occur in tlie same specimen. 



In the recent Chilostomata, according to Calvet, Levinsen, and others, 

 sonic gciici'a lia\c a double and some a single wall, and tlu>re is no impor- 

 tant classiticatory \alue to this difference. If the differences above de- 

 scribed are due primarily to the steepness of pitch of the wall laminas, it 

 is liT\cly that the classificatory value of this phase of wall structure in the 

 Treposfomata is also of subordinate rank. The calcareous lamina^ are 

 laid down in the growing edge of the wall j)arall('l to the two surfaces of 

 the amalgamated ectosarc, as shown in figui'cs 50 ajid 57. If the wall is 

 knife-edged, as shown in figure 56, the laminae will pitch very steeply 

 and there will be a sharply defined central zone. If the growing edge of 

 the wall is blunt, as shown in figure 57, there will be no definite median 

 boundary. Phases intermediate between these two extremes will also he 

 of common occurrence. 



It has often been asserted that the duplex character of trepostome 

 walls is proven by the tendency of the walls to split down the middle. 

 If this were a fact, it would unquestionably be a good argument; Init it 

 is not a fact, ^^'e have examined thousands of fractures, under high 

 magnifications, and have found that the split invariably folloAvs the direc- 

 tion of the laminffi, crossing back and forth across the median region of 

 file wall with perfect indifference. AVhere the lamina? are very steep, the 

 wall often appears under low magnification to l)e split accurately in the 

 middle; but a closer examination will always show that it is not. In the 

 axial region, where the lamintu are parallel with the surface of tln' wall 

 (figiu'e 58. a.i'). tlie split may follow the median line, or any other line 

 parallel with it, often for a considerable distance. The highest obtain- 

 able magnification (oil immersion) has failed to reveal any indication of 

 duplex structure of th<' walls in the axial icgion of the zoarium. The 

 whole wall iimler such magnification appears merely as parallel chains 

 of granules. Figure 58 shows the wall of HiujindleUa spinosa where it 

 emerges from the axial region. Op])osite a.r there appears to be a dark 

 di\idiiig line, but fai'tliei' in toward the axial region this entirely disap- 

 pears. Farther out toward the mature region the dark median /.one be- 

 comes interrupted and tlie lamimi! bend more or less smoothly over the 

 axial region of the wall. The mature region of the wall of this genus is 

 like that of JJehai/ia, figure 50. 



THE OINGVLUM 



'r\\{: walls of thr Trrpostomata often show a greater or less amount of 

 secondary thickrnini;-. These secondary deposits we have designated the 



