GENERA OF CHyETETID^E AND MONTICULIPORIDyE. 295 



the larger corallltes, and generally subpolygonal or angular in 

 shape. Large corallites with few and remote tabulae ; small 

 corallites closely tabulate. 



Obs. — This species is so well known that I need only make 

 a few remarks upon its minute structure, as elucidated by thin 

 sections. Both tangential and vertical sections (the latter taken 

 at right angles to the plane of the frond) show that the coral- 

 lum is really dimorphic ; though the mere examination of the 

 surface with a lens, except in occasional specimens, would lead 

 one to conclude that small tubes are almost wanting. Tangen- 

 tial sections (PI. XIII., figs, i and i a) show, however, that 

 there exists really a fair number of small tubes scattered among 

 the larger corallites in an indiscriminate manner ; and vertical 

 sections (PI. XIII., fig. 1 b) prove that these are really special 

 corallites, as they are much more closely tabulate than the 

 ordinary corallites. There seems also generally to exist a very 

 small number of thickened spiniform tubes, easily recognised 

 in tangential sections by their circular outline and thick dark 

 margins ; though I have not been able to detect spines on the 

 surface. The corallites which occupy the " mamelons " cer- 

 tainly look a little larger, as a rule, than the average ; but as 

 they cannot be recognised in thin sections, I suspect this is an 

 illusory appearance, due to their greater nearness to the eye of 

 the observer. Their calices may be open, or may be closed by 

 a thin calcareous membrane, and they often have minute tubes 

 intercalated among them. The walls of the corallites, both 

 large and small, are slightly but unmistakably thickened by 

 a deposit of light-coloured sclerenchyma, which increases in 

 amount as the calices are approached (PL XIII., fig. i a) ; but 

 in the limited extent of thickening, as well as in the compara- 

 tively small number of the smaller corallites, M. mamimtlata 

 shows itself to be not nearly such a characteristic member of 

 the section Heterotrypa as M. rajjiosa, D'Orb., M, frondosa, 

 D'Orb., M. Jamesi, Nich., or M. tnmida, Phill. 



Formation and Z<3fa/zVj.— Abundant in the Cincinnati group, 

 Cincinnati, Ohio. 



