Prof. Nicholson oii Clctssification of Tabulate CoraUr 101 



Silurian genus Tetradium, Dana, in which the corallum is 

 massive, and is composed of polygonal corallites, of great 

 length, and in close contiguity. There are no " mural pores ; " 

 the tabulae are mmierous and complete; and there are 

 generally four septa, which look as if they were formed by 

 inflections of the wall. 



Tetradmm resembles Halysites in its long imperforate 

 corallites, and occasionally in its mode of growth, while its 

 peculiar septa, except in number, remind one of the " pseudo- 

 septa " of the Helioporidm. Upon the whole, it seems pro- 

 bable that this curious genus should be referred to the 

 Alcyonaria, though I know of no group in this order to 

 which it could be definitely referred. 



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

 Silurian genus Tliecia, E. 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 vertical tubuli. A few 

 blunt septal ridges are present in each corallite ; the tabulse 

 are well developed ; and the visceral chambers of contiguous 

 tubes communicate by horizontal canals. 



This extraordinary genus forms in many respects a Irak 

 between the Perforate corals and the Alcyonarian family of 

 the Helio2ooridw. It resembles the former in the fact that 

 the visceral chambers of contiguous corallites communicate 

 directly with one another; and it approaches the latter in 

 the fact that the ordinary poljrpes are surrounded by what 

 has usually been regarded as a tubular " ccenenchyma," 

 though not really of this nature. Judging from the analogy 

 of the recent Helio]pora, the so-called " coenenchymal tubuli " 

 are really tenanted by special zooids, and cannot, therefore, 

 be properly coenenchymal. In the possession, then, of a 

 series of large polypes, surrounded by a much more numerous 

 series of small specialised polypes, Thecia agrees with Helio- 

 'pora and Heliolites ; and the genus, therefore, is probably 

 Alcyonarian. It differs from the Helioporidce, however, in 

 the presence of distinct and direct channels of communication 



