24 TABULATE CORALS. 



single intersecting series, as in Halysites, and Billings has 

 figured similar forms (Geol. of Canada, fig. 71, b). Professor 

 Safford regarded the genus as referable to the Rugosa, on 

 account of the quadripartite character of the septa ; but the 

 relations of the genus to Halysites were pointed out by Mr 

 R. Etheridge, jun., and myself (Ann. Nat. Hist, sen 4, 

 vol. XX. p. 163). It approaches Halysites in its long tubular 

 corallites, its imperforate walls, and occasionally in its mode 

 of growth ; but its peculiar septa, and the want of smaller tubes 

 among the larger ones, would show it to be the type of a 

 special group. Its septa are much more like those oi Heliolites 

 than of Halysites, looking as if they were formed by inflections 

 of the wall inwards along its whole length ; and I think that the 

 genus will probably have to be referred to the Alcyonaria, 

 though I know of no group of this order to which it could be 

 definitely referred. 



IX. Thecid^. — This group comprises only the singular 

 Silurian genus T/iecia, Edw. and H., the corallum of which 

 forms laminar expansions, covered below by an epitheca, and 

 having the calices placed upon the upper surface. The 

 tubular corallites cannot be said to be bounded by distinct 

 proper walls ; but they are embedded in and surrounded by 

 a dense tissue composed of minute polygonal vertical tubuli, 

 which normally open on the surface by very small and irreg- 

 ular, often stelliform, apertures. A few blunt septal ridges 

 (five to ten in number) project into the visceral chambers of 

 the polypes, which are crossed by irregular but well-developed 

 tabulae, and likewise often communicate with neighbouring 

 tubes by means of canals passing horizontally through the 

 intervening tissue. The smaller tubuli, which separate the 

 larger corallites, do not appear to be distinctly tabulate. 



The structure of this genus is so extraordinary that it must 

 clearly form the type of a special group, as it was made to do 

 by Milne-Edwards and Haime. Nor need we wonder that its 

 anatomy has been so little comprehended, seeing that its real 

 structure can only be made out by means of microscopic sec- 



