.V UB- GEA' US HE TER O TR YPA . 



129 



Britain. So far, in fact, as its external characters are con- 

 cerned, it is very like M. pulchclla, resembling it especially 

 in the existence of clusters of thin-walled polygonal corallites, 

 interspersed at short intervals among similarly- shaped but 

 slightly smaller tubes. In the absence, therefore, of any 

 accurate microscopic knowledge of the internal structure of 

 the two forms, it was almost inevitable that they should have 

 been grouped together, though their minute characters, as will 

 appear hereafter, are very different. 



Superficially, M. Andrewsii is readily recognised by its sub- 

 equal, polygonal, or sub-polygonal calices, with the numerous 





Fig. 21. — Minute structure of M. Andrcci'sii, Nich., from the Cincinnati Group of Ohio. A, 

 Tangential section enlarged eighteen times, and B, part of the same enlarged fifty times, 

 showing the structure of the wall, and the two sets of corallites ; c, Vertical section, 

 enlarged eighteen times, showing the different tabulation of the large and small corallites ; 

 D, Part of the axial region of a transverse section, enlarged eighteen times, showing the 

 thin-walled and polygonal condition of the tubes in the centre of the branches. 



little clusters of slightly larger-sized apertures. Intermingled 

 with the ordinary calices, and especially at their angles, the 



I 



