GENERAL AND COMPARATIVE STRUCTURE. 



37 



M. pitlchella, E. and H. (fig. i, c), M. petasiforjnis, Nich. 

 (fig. 2, b), M. Girvanensis, Nich., M. Treiitonensis, Nich., M. 

 Jamesi, Nich., &c. The actual thickness of the proper wall 

 of each tube, internal to the dark boundary-line which marks 

 the primordial wall, is very variable, being exceedingly limited 



Fig. I. — A, Tangential section of a few corallites of the typical MonticuUpora petropolitaiia, 

 Pand., from the Lower Silurian of Sweden ; B, Tangential section of a corallite of a typi- 

 cal example of Montkidipora ramosa, E. and H., from the Cincinnati group of Ohio ; c, 

 Tangential section of a corallite of a typical example of Montiailipora pulchclla, E. and H., 

 from the Wenlock Limestone of Dudley ; D, Tangential section of a corallite of the typi- 

 cal Chcvtetes radians, Pand., of the Carboniferous rocks of Russia. All the sections are 

 taken just below the calices : A, B, and C are enlarged fifty times ; D enlarged twenty- 

 five times. 



in forms like M. petasiforviis, Nich. (fig. 2, b), while it is more 

 extensive in forms like M. pulchella, E. and H., and is some- 

 times exceptionally great, as in types like M. Treitionensis, 

 Nich. The internal secondary wall also is usually, though not 

 always, rounded off internally, so as to give the whole tube a 

 circular or oval aspect, while the thin line of the original wall 

 is almost always markedly polygonal. 



In another group of Montiailipo7'a;, represented by forms 



