Stromatoporoids and of Eozoon. 



449 



The formation of a new 

 upwards from an original 



systems already in existence. 



lamina is due to the budding 



central or " interpolated central " chamber, the bud beconiin*^ 



the centre of a new spiral. Tlie peripiierul series of cliambers 



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



from a spiral to a cyclical plan. 



The small central clianiber has a narrow straight neck, 

 whence originates the circumambient chamber, and the latter 

 continues the spiral and gives otf radial offslioots. The 

 sarcode following the circumambient chamber becomes seg- 

 mented, owing to the formation of radial vertical j)artitions, 

 and the outer walls increase in thickness. 



Fi-. 3. 



CUtthrodictijon striatelhan, d'Orb. Wenlock. a, interpolated ceutrill 

 cli amber. X 30. 



A varying degree of complexiry 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 witii its system of 

 spirals, and each latilamina marks a season of growth. 



A vertical section of an Ac.tinostromac\\iB.\\\\ou^\\ so many 

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

 limes a central chamber produces a wart or tubercle. 



The branched forms {Idiostroma^ Stachyodes) have only 

 vertical budding along one axis. 



