VIl] 



OF HEXAGONAL SYMMETRY 



325 



'We have many varied examples of this principle among corals, 

 wherever the polypes are in close juxtaposition, with neither 

 empty space nor accumulations of matrix between their adjacent 

 walls. Favosites gothlandica, for instance, furnishes us with an 

 excellent example. In the great genus Lithostrotion we have some 

 species that are "massive" and others that are "fasciculate" ; in 

 other words in some the long cylindrical corallites are in close con- 

 tact with one another, and in others they are separate and loosely 

 bundled (Fig. 127). Accordingly in the former the corallites are 



Fig. 127. Lithostrotion Martini. 

 (After Nicholson.) 



Fig. 128. Cyathophyllum hexagonum. 

 (From Nicholson, after Zittel.) 



squeezed into hexagonal prisms, while in the latter they retain their 

 cylindrical form. Where the polypes are comparatively few, and 

 so have room to spread, the mutual pressure ceases to work or 

 only tends to push them asunder, letting them remain circular in 

 outline (e.g. Thecosmiha). Where they vary gradually in size, as 

 for instance in Cyathophyllum hexagonum, they are more or less 

 hexagonal but are not regular hexagons ; and where there is greater 

 and more irregular variation in size, the cells will be on the 

 average hexagonal, but some will have fewer and some more sides 

 than six, as in the annexed figure of Arachnophyllum (Fig. 129). 



