GENERA OF FA VOS/TIDAi. 



49 



The general shape of the corallum of Favositcs Gothlaiidica 

 is very variable ; and its size differs still more either in speci- 

 mens of different ages, or in specimens derived from different 

 formations. Typical examples, from the Upper Silurian, are 

 generally more or less hemispherical or discoidal, with a de- 

 pressed or slightly convex upper surface (fig. 14, b), the cor- 

 allum being attached to some foreign body by the centre of 

 its base, which is sometimes drawn out into a short peduncle. 

 The lower surface is also covered with a thin, smooth, or con- 

 centrically-striated epitheca, which is commonly worn ofT in 

 old specimens, though sometimes greatly thickened. Average 

 examples of this kind are usually two or three inches in dia- 

 meter, sometimes more, with a height of three quarters of an 

 inch to an inch ; but I have figured a small example — the 

 youngest I have seen — from the Wenlock Limestone of 

 Dudley, in which the diameter is only about eight lines, with 

 a height of three lines (PI. I., fig. 3). In the Devonian, the 

 species seems to have assumed a much greater luxuriance of 

 growth, examples of much larger size and of very variable form 

 being far from uncommon. Even the larger specimens, how- 

 ever, seem to preserve the same general type — namely, that of 

 a flattened or hemispherical expansion, attached by a portion of 

 its base, and having its inferior surface covered by an epitheca. 



The corallites in the typical Upper Silurian examples of F. 

 Gothlandica are prismatic, thin-walled, usually pentagonal or 

 hexagonal, and very regular in form and size in any given 

 example {see fig. 14, a and b, and PI, I., figs, i and 2). The 

 calices, of course, have a corresponding polygonal form and 

 regularity of outline ; and here we seem to reach one of the 

 most distinctive characters of the species. It need hardly be 

 said that in no specimen are the corallites absolutely uniform 

 in size or form. In every individual example, the corallum 

 consists of younger and older, and therefore of smaller and 

 bigger, tubes, and the form of these necessarily varies with 

 variations in the pressure to which each is subjected by its 

 neighbours. Still, the average corallites of any particular speci- 



