Stromatoporoids and of Eozoon. 



440 



systems alroody in existence. The formation of a new 

 lamina is due to the buddin^? upwards from an original 

 central or '' interpolated central " cliamber, the bud becoming 

 the centre of a new spiral. The peripheral series of chambers 

 may become C3'clical. In Orhitolites there is often a change 

 from a spiral to a cyclical plan. 



The small central chamber has a narrow straight neck, 

 whence originates the circumambient chamber, and the latter 

 continues the spiral and gives off radial offshoots. The 

 sarcode following the circumambient chamber becomes seg- 

 mented, owing to the formation of radial \ertical partitions, 

 and the outer walls increase in thickness. 



Fio-. 8. 



Clathrodictyon striatellum, d'Orb. Wenlock. 

 chamber, x 30. 



a, interpolated central 



A var} ing degree of complexity is brought about by the 

 pressure of neighbouring spiral systems on each other and 

 by the curving of successive laminae. Each lamina represents 

 so many vertically produced buds^ each with its system of 

 spirals^ and each latilaniina marks a season of growth. 



A vertical section of an Actl7iostromac\\i%i\\\ou^\\ so many 

 coils, each coil being in relation to a central chamber. Some- 

 times a central chamber produces a wart or tubercle. 



The branched forms {Idiostroma^ Stachyodes) have only 

 vertical budding along one axis. 



