GENERA OF FA VOSITID&. 



59 



tionate number of average corallites ; the small tubes are fewer 

 in number and not so conspicuous ; and the large tubes open 

 on the surface by actually smaller apertures, the diameter of 

 the larger calices being generally about one line, but some- 

 times rather less. Still it would be impossible to confound 

 such examples with specimens of F. Gothlandica, since the 

 observer has no difficulty in at once recognising the presence 

 of a certain number of tubes, which catch the eye in conse- 

 quence of their being larger than the others ; while the pris- 

 matic form of the corallites so characteristic of the latter 

 species is never regularly developed, and the bigger calices 

 have a distinctly circular outline. As regards the internal 

 structure of this form there is little of importance to note. 

 The presence of septa in the form of short spines, vertical 

 striae, or tubercular ridges, can usually be made out by ex- 

 amining the surface with a lens ; and thin transverse sections 

 almost invariably confirm the existence of these structures (PI. 

 I., fig. 7, and PI. II., fig. i b). These septal spines or ridges 

 are, however, always short, and extend but a short distance 

 inwards into the visceral chamber ; nor would they usually be 

 recognised at all by a mere naked-eye examination of the sur- 

 face. Vertical sections (PI. II., fig. i a} show the tabulae to be 

 complete, slender, and slightly flexuous, placed at variable dis- 

 tances apart, six or seven usually occupying the space of two 

 lines. It is specially worthy of note in this connection that 

 Milne- Edwards and Haime have figured an example of this 

 species from the Wenlock Limestone of Wenlock (Brit. Foss. 

 Corals, PL LX., fig. 2/), in which the interior of the tubes is 

 roughened by elevated points or ridges, which, though badly 

 figured, entirely correspond in appearance to the " squamae " 

 which we shall see to be present in F. Forbesi, var. tuberosa, 

 Rom., from the Devonian of North America, and which I 

 believe to be unquestionably the same in their nature. 

 Whether similar rudimentary tabulae are present at all com- 

 monly in the Upper Silurian forms of F. Forbesi, I cannot 

 say ; for these structures very rarely reveal themselves in thin 



