THE GRAPTOLITES AND CORALS 305 



median section of a corallite this is seen to be a vertical 

 pillar extending the whole length. It is crossed by 

 numerous very regular tabulae, which are very slightly 

 convex upwards except near the columella, where each 

 is lifted up into a cone, and towards the margin, where it 

 falls steeply into the marginal zone of dissepiments. In 

 the calyx of an adult corallite the septa appear very 



FlG. 92. LlTHOSTROTION IRREGULARE (PHILLIPS), VlSEAN 



(CARBONIFEROUS LIMESTONE). 



a, Small portion of a young corallum. (x.) b, Vertical section of a 

 corallite. ( X 2.) c, Columella ; P.Z., peripheral zone of dissepiments. 

 c, Transverse section of a full-grown corallite. (X3-) d, Transverse 

 section of a young corallite. ( X (j.) H, Cardinal ; G, counter ; A , A , 

 alar septa, a, b, c, After Milne-Edwards and Haime ; d, original. 



perfectly radial. The compression of the columella, 

 however, indicates a median plane, and on examining a 

 cross-section of the younger part of a corallite, a 

 distinctly bilateral arrangement can be recognized, and 

 the same proto-septa as in Zaphrentis can be made out 

 (Fig. 92, d). 



In other species of Lithostrotion (e.g., L. martini, which 



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